Electrophysiologically mature cardiomyocytes and methods for making same

ABSTRACT

Methods to prepare electrophysiology mature cells from immature cells are provided, without the need for genetic manipulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/501,034, filed Jan. 2, 2019, which is a continuation of U.S.application Ser. No. 14/615,291, filed Feb. 5, 2015, which claims thebenefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser.No. 61/936,807, filed Feb. 6, 2014, the contents of each of which areincorporated by reference in their entireties.

STATEMENT OF GOVERNMENT SUPPORT

This invention was supported in whole or in part under Grant Number RO1HL72857 awarded by National Institutes of Health (NIH). Accordingly, theU.S. government has rights to the inventions disclosed herein.

BACKGROUND

Throughout this disclosure, various publications, patents and publishedpatent specifications are referenced by an identifying citation. Alsowithin this disclosure are Arabic numerals referring to referencedcitations, the full bibliographic details of which are providedimmediately preceding the claims. The disclosures of these publications,patents and published patent specifications are hereby incorporated byreference into the present disclosure to more fully describe the stateof the art to which this invention pertains.

Loss of non-regenerative, terminally differentiated cardiomyocytes (CMs)is irreversible; myocardial repair is further hampered by a severeshortage of donor cells and organs. CMs can be differentiated from human(h) embryonic stem cells (ESCs) that can propagate indefinitely inculture while maintaining their pluripotency (Kehat, I. et al. (2001)The Journal of Clinical Investigation 108:407-414; Mummery, C. et al.(2003) Circulation 107:2733-2740; Pera, M. F. et al. (2000) Journal ofCell Science 113(Pt 1):5-10; Thomson, J. A. et al. (1998) Curr. Top Dev.Biol. 38:133-165; Xue, T. et al. (2005) Circulation 111:11-20; Thomson,J. A. et al. (1998) Science 282:1145-1147; Reubinoff, B. E. et al.(2000) Nat. Biotechnol. 18:399-404; Xu, C. et al. (2002) Circ Res.91:501-508; He, J. Q. et al. (2003) Circ. Res. 93:32-39). Therefore,hESCs can provide an unlimited ex vivo source of CMs for clinicalapplication and drug testing. While existing efforts mostly focus on thederivation of heart cells from hESCs, it is imperative that thesederived CMs are functionally mature in ways similar to their adultcounterparts before the desired therapeutic outcome can be achieved. Infact, hESC-CMs exhibit embryonic- or fetal-like electrophysiologicalproperties (Mummery, C. et al. (2003) Circulation 107:2733-2740; He, J.Q. et al. (2003) Circ. Res. 93:32-39; Satin, J. et al. (2004) J.Physiol. 559:479-496). For instance, hESC-derived ventricular CMsexhibit spontaneously firing action potentials (AP), in contrast to thenormally quiescent-yet-excitable phenotype of adult. Indeed, it waspreviously demonstrated that transplantation of a node ofelectrically-active hESC-CMs, consisting of a mixture of ventricular,atrial and nodal cells, could collectively serve as a surrogatepacemaker in vitro and in vivo (Xue, T. et al. (2005) Circulation111:11-20). Thus, immature hESC-CMs are potentially arrhythmogenic aftertransplantation. Moreover, there is a need to develop protocols forrescuing the immature phenotypes for their eventual clinical and otherapplications (e.g., cardiotoxicity screening and heart disease modelsthat accurately reflects the adult heart). This disclosure satisfiesthis need and provides related advantages as well.

SUMMARY

In one aspect, this disclosure provides methods to mature theelectrophysiological phenotype of a cell, a population of cells, and/ora tissue by contacting or applying to the cell or tissue an electricalcurrent, i.e., electrical pacing. The phenotype of the cardiac musclecell produced by the method comprises the five phases of a cardiacaction potential; specifically, they are quiescent-yet-excitable,without Phase 4 depolarization or spontaneous firing, and maintain arelatively negative resting potential (at ˜−80 mV).

The disclosure also provides methods for treating a patient in needthereof, wherein the patient is in need of one or more of promotingfunctional integration of these cells with the recipient heart aftertransplantation, thus providing therapeutic benefit such as to eliminateor reduce the arrhythmogenicity of immature cells and/or cardiomyocytesdue to the immature electrophysiology of the cell or other resultantelectrical disturbances. This disclosure also provides in vitrocompositions and methods for drug discovery, cardiotoxicity screeningand disease modeling, as the matured cells or their engineered tissuescan more accurately reflect the properties of the native adult heart.

In one aspect, the disclosure provides a method for inducing expressionof the Kir2.1 gene in a cell lacking functional Kir2.1 gene expression,with the proviso that the method excludes genetic manipulation of thecell, comprising, or alternatively consisting essentially of, or yetfurther consisting of, administering to the cell an effective amount ofelectrical pacing, thereby inducing expression of Kir2.1 gene expressionin the cell.

In another aspect, a method is provided for inducing anelectrophysiologically mature phenotype in a cell that is not terminallydifferentiated and/or electrophysiologically mature, comprising, oralternatively consisting essentially of, or yet further consisting of,administering to the cell that is not terminally differentiated and/orelectrophysiologically mature an effective amount of electrical pacing,thereby inducing an electrophysiologically mature phenotype in the cell.As used herein, the term electrophysiologically immature phenotypeintends a cell that displays one or more of a degree of automaticity(i.e., repetitive or spontaneous firing of AP at least 0.5 1, 2, 3 Hz orhigher, Phase 4 depolarization, a relatively positive resting membranepotential or MDP of more positive than −80 mV, and delayedafter-depolarization). In one aspect, the method excludes geneticmanipulation of the cell to enhance or increase Kir2.1 expression.

In another aspect, a method is provided for preparing anelectrophysiologically mature cardiomyocyte from a cell that is notterminally differentiated, comprising, or alternatively consistingessentially of, or yet further consisting of, administering to the cellthat is not terminally differentiated, an effective amount of electricalpacing, thereby preparing an electrophysiologically maturecardiomyocyte. In one aspect, the method excludes genetic manipulationof the cell to enhance or increase Kir2.1 expression.

Cells that can be used in the method include cells that are notterminally differentiated, e.g., an embryonic stem cell, a pluripotentstem cell, an embryoid body, a mesodermal cardiosphere or an inducedpluripotent stem cell, or their cardiac derivatives.

In one aspect, the electrophysiologically mature cardiomyocyte exhibitsone or more of a decrease of the atrial natriuretic factor (ANF) and/oran increase in one or more of the contractile proteins MHCα, MHCβ, MLC2aand MLC2v as well as cardiac calcium ion channel proteins. Methods todetect and monitor expression of these markers are described herein, orare known in the art and described in U.S. Patent ApplicationPublication Nos. 2008/0089874 and 2012/0014924, each incorporated byreference herein.

In another aspect, the electrophysiologically mature cardiomyocytecomprises a cardiac cell of the group of a ventricular cell or an atrialcell. The cell is a mammalian cell, e.g., a murine cell, a bovine cell,a canine cell, a feline cell, an equine cell or a human cell.

In any of the above methods, the electrical pacing comprises applying oradministering a pulsed electrical current of about 2.0 v/cm to about 3.0v/cm, or alternatively about 2.5 v/cm, for about 2 msecs to about 8msecs, or alternatively about 5 msec at a width of about 0.5 HZ to about1.5 HZ, or alternatively about 1 HZ. In any of the above methods, theelectrical pacing comprises a pulsed electrical current of about 1, 2.5,10, etc. v/cm, for about 2, 5, 10 msecs, etc. at a frequency of 1, 2, 3,10 HZ etc. for various durations (1, 3, 5, 7 days, 2, 3, 4 weeks, etc.).

Further provided is an isolated cell prepared by the method as describedabove herein. In one aspect, the cell can be expanded or cultured toproduce a population of cells that in one aspect, are substantiallyhomogenous. In a further aspect, the substantially homogenous populationis a clonal population. After the cell has been modified by a method asdisclosed herein, it may be expanded to a substantially homogenouspopulation (e.g., a clonal population) of these cells or alternatively,differentiated to a more mature cell type. Compositions containing thesecells and populations of cells are also provided by this disclosure.

Thus, in one aspect, this disclosure provides an isolatedelectrophysiologically immature cell, such as an embryonic stem cell,pluripotent stem cell or an iPSC, or their derivatives, that has beenmodified to provide a mature electrophysiological phenotype. Thedisclosure also provides for a clonal population or a population ofcells differentiated from electrophysiologically immature cells toprovide a mature electrophysiological phenotype. In one aspect, thecells or tissues are further characterized by not being modified in oneor more of the following manners: by transduction of a polynucleotidethat promotes or inhibits I_(K1) activity of the cells; by transductionof a polynucleotide that modulates Kir2 and HCN protein expression; bytransduction of a polynucleotide that encodes a Connexin protein orenhances the expression of a Connexin protein; by transduction of apolynucleotide that modifies the calcium handling properties such ascalsequestrin (CSQ); and by transduction of a polynucleotide thatmodifies other critical electrophysiological activities of the cellssuch as I_(Kr), I_(Ks), I_(Na), I_(Ca), I_(tol), I_(NaCa), I_(NaK) andI_(pCa). Methods to detect and monitor expression of these markers areknown in the art and described in U.S. Patent Application PublicationNos. 2008/0089874 and 2012/0014924, each incorporated by referenceherein.

These cells, populations and compositions have therapeutic anddiagnostic uses. Non-limiting therapeutic uses include regeneratingcardiac tissue, improving cardiac function, restoring action potentialof cardiac tissue; and treating or preventing cardiac malfunction. Thesemethods can be achieved by administering an effective amount of a cellor population of cells or tissue to a host in need thereof, e.g., amammal such as a murine, a bovine, an equine, a canine, a feline, or ahuman patient. The cells and population of cells can be useddiagnostically to screen drug or other therapeutic candidates.

The cells and compositions of the present invention can be used in themanufacture of medicaments and for the treatment of humans and otheranimals.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1F show electrophysiology of pluripotent stem cell derived CMs.FIG. 1A shows representative tracings of spontaneously firing (left) andquiescent (right) ventricular (top), atrial (middle) and pacemaker(bottom) action potentials (APs) recorded from ddhESC-CMs. The arrowsdenote a phase 4-like depolarization, which can be a proarrhythmicsubstrate of automaticity. FIG. 1B shows the percentage distribution ofventricular, atrial and pacemaker phenotypes, and FIG. 1C shows thepercentage distribution of spontaneously-firing and quiescent ddhESC-CMs(n=70). FIG. 1D shows occurrence of DAD in ddhESC-CMs. FIG. 1E showsrepresentative current tracings of I_(CaL), I_(Na), I_(K1) and I_(f),I_(Kr) and I_(Ks) in ventricular ddhESC-CMs. FIG. 1F showsrepresentative tracings of spontaneously firing (left) and quiescent(right) ventricular and atrial human iPSC-CMs.

FIGS. 2A-2F show current-voltage, activation and inactivationrelationships of currents in ddhESC-CMs. FIG. 2A shows current-voltage(I-V) relationships of I_(f) in ventricular (n=6) and atrial (n=6)ddhESC-CMs. FIG. 2B shows steady-state activation curves of I_(f) inventricular (n=6) and atrial (n=6) ddhESC-CMs. FIG. 2C shows I-Vrelationships of I_(CaL) in ventricular (n=17) and atrial (n=7)ddhESC-CMs. FIG. 2D shows steady-state inactivation curves of I_(CaL) inventricular (n=17) and atrial (n=7) ddhESC-CMs. FIG. 2E shows I-Vrelationship of I_(Na) in ventricular ddhESC-CMs (n=4). FIG. 2F showsI-V relationships of I_(Kr) in ventricular (n=6) and atrial (n=3)ddhESCCMs.

FIGS. 3A-3D show electrophysiological properties of hESC-CMs derivedfrom the HES2 and H1 lines. FIG. 3A shows representative actionpotential (AP) waveforms of spontaneous and quiescent ventricular,atrial and pacemaker END2-hESC-CMs as indicated (left), showingpercentage distribution of ventricular, atrial and pacemaker phenotypes(top left), and the percentage distribution of spontaneously firing vs.quiescent END2-hESC-CMs (bottom left; n=33). The arrows indicate a phase4-like depolarization, which can be a pro-arrhythmic substrate forautomaticity. FIG. 3B shows representative current (I-V) tracings ofI_(f) and I_(K1) in ventricular END2-hESC-CMs. FIG. 3C shows I-V curvesof I_(f) in ventricular (n=4) and atrial (n=4) END2-hESC-CMs. FIG. 3Dshows representative AP waveforms of ventricular, atrial and pacemakerEB-hESC-CMs (top), current tracings of I_(f) and I_(K1) in ventricularEB-hESC-CMs (bottom left) and the percentage distributions ofventricular, atrial and pacemaker phenotypes, and spontaneously firingvs. quiescent EB-hESC-CMs (bottom right; n=18).

FIGS. 4A-4G show effects of I_(K1) presence on electrophysiology ofpluripotent stem cell derived-CMs. FIG. 4A shows in silico analysis ofI_(K1) effects on the maturation of ventricular embryonicelectrophysiological phenotypes. FIG. 4B shows action potentials (APs),I_(f) and I_(K1) of Ad-Kir2.1-transduced ventricular ddhESC-CMs. FIG. 4Cshows APs, I_(f) and I_(K1) of Ad-Kir2.1-transduced ventricularEND2-hESC-CMs. Ad-Kir2.1 transduction eliminated the presence of phase4-depolarization in both cell types (arrows). DAD was also neverobserved in Ad-Kir2.1-transduced cells. FIG. 4D shows the percentage ofquiescent ventricular ddhESC-(n=45) and END2-hESC-CMs (n=20) increasedsignificantly to 100% after Ad-CGI-Kir2.1 transduction (n=7 and n=13).FIG. 4E shows resting membrane potentials (RMPs) of Ad-Kir2.1-transducedCMs became significantly hyperpolarized relative to control andcomparable to healthy adult CMs. FIG. 4F shows Ad-Kir2.1-silencedventricular hESC-CMs spontaneously fired APs upon the addition of 200 μMBa²⁺, a potent I_(K1) blocker, indicating I_(K1) is a dominant regulatorin controlling the automaticity of hESC-CMs. FIG. 4G shows the mRNAexpression of contractile elements is significantly reduced afterAd-Kir2.1 transduction relative to control hESC-CMs (n=3).

FIGS. 5A-5D show Kir2.1 overexpression confers upon atrial ddhESC- andEND2-hESC-CMs the electrophysiological properties of mature CMs.Representative APs, I_(f) and I_(K1) of Ad-Kir2.1-transduced atrialddhESC-CMs (FIG. 5A) and atrial END2-hESC-CMs (FIG. 5B). In both celltypes, Ad-Kir2.1 transduction eliminated the appearance of phase4-depolarization (arrows). FIG. 5C shows resting membrane potentials(RMPs) of Ad-Kir2.1-transduced atrial ddhESC- (n=4) and END2-hESC-CMs(n=9) were significantly hyperpolarized compared to untransducedcontrols (n=42 and n=1). FIG. 5D shows current-voltage (I-V) curve ofI_(K1) in Ad-Kir2.1-transduced atrial and ventricular ddhESC- (n=4 andn=4) and END2-hESC-CMs (n=5 and n=4). I-V relationships of thetransgenic Kir2.1 are similar between the atrial and ventricular CMs ofeach differentiation method.

FIGS. 6A-6G show effects of electrical conditioning on ddhESC-CMs. FIG.6A shows electrophysiology of electrically conditioned ddhESC-CMs.Action potential (AP) profiles (left) of electrically conditioned atrialand ventricular ddhESC-CMs showed the absence of phase 4-deloparization(n=11). Both cell types also displayed more hyperpolarized restingmembrane potential (RMP) (middle) that have resulted from anelectrically induced increase in Kir2.1 expression (right). FIG. 6Bshows electrically induced Ca²⁺ transients and Ca²⁺ wavefronts fromunpaced control (top left; n=20) and electrically conditioned ddhESC-CMs(bottom left; n=34) and their average peak amplitude (right). FIG. 6Cshows caffeine-induced Ca²⁺ transients from unpaced control (top, black;n=23) and electrically conditioned ddhESC-CMs (bottom, gray; n=17) withtheir average peak amplitude (right; control, left bar, conditioned,right bar). FIG. 6D shows transcript analysis of Ca²⁺-handling proteins(calsequestrin (CSQ), voltage-gated L-type Ca²⁺ channel ordihyropyridine receptor (DHPR), junctin (Junct), phospholamban (Phlmb),ryanodine receptor (RyR), sarco/endoplasmic reticulum Ca²⁺ ATPase(SERCA), and triadin (Trdn)) showed significant increase for CSQ, Junctand Trdn (n=5). FIG. 6E shows mRNA expression of the t-tubule biogenesisproteins caveolin-3 (Cav3) and amphiphysin-2 (Amp2)) and adultatrial-specific marker (atrial natriuretic factor (ANF)) (n=5). FIG. 6Fshows mRNA expression of contractile elements (myosin heavy chain(MHC)α, MHCβ, myosin light chain (MLC)2a, and MLC2v) (n=5). All bargraphs shown as mean±SEM with * denoting statistical significance(p<0.05). FIG. 6G shows typical TEM images of myofilaments in unpaced(n=15) and electrically conditioned (n=13) ddhESC-CMs showing distinctz-lines (arrows).

FIG. 7A shows effect of the duration (7, 14 and 21 days) of electricalstimulation at 1 Hz on Kir2.1 expression. Transcript expressionnormalized to GAPDH, and normalized to time-matched non-stimulatedcontrol (broken line)(n=9, from 3 independent batches). FIG. 7B showseffect of the duration as in FIG. 7A, but performed on later-stage(40-50 days old) hESC-CMs. FIG. 7C shows effect of cessation ofelectrical conditioning of early-stage ddhESC-CMs. Broken line indicatesthe level of continuous stimulation over the same period. FIG. 7D showsthe effect of frequency.

FIGS. 8A and 8B show immature electrophysiological properties of mouse(m)ESC-CMs. FIG. 8A shows representative tracings of spontaneouslyfiring (left) and quiescent (right) ventricular (top), atrial (middle)and pacemaker (bottom) APs of mESC-CMs. The arrows indicate a phase4-like depolarization, a proarrhythmic substrate of automaticity. FIG.8B shows the percentage distribution of ventricular, atrial andpacemaker phenotypes (left) and the percentage distribution ofspontaneously firing vs. quiescent characteristics (right) of total ofmESC-CMs (n=60).

FIGS. 9A-9F show Kir2.1 overexpression confers upon mouse (m)ESC-CMs theelectrophysiological properties of mature CMs. FIG. 9A shows arepresentative tracing of control ventricular mESC-CMs with DAD (left)and after Ad-Kir2.1 transduction showing absence of a phase4-depolarization (right, arrow). Arrow heads indicate time of electricalstimulation. FIG. 9B shows total, Ba²⁺-insensitive I_(f) andBa²⁺-sensitive I_(K1) currents recorded from the same cells in FIG. 9A.FIG. 9C shows current-voltage (I-V) relationships of I_(K1) recordedfrom control spontaneously firing (open circles; n=3), control quiescent(solid circles; n=5), Ad-Kir2.1-transduced ventricular mESC-CMs (solidtriangles; n=4) and adult mouse ventricular CMs (open triangles; n=5).FIG. 9D shows the zoomed-in outward components of I_(K1) from FIG. 9C.FIG. 9E shows Kir2.1-overexpression (n=15) rendered nearly allventricular mESC-CMs quiescent compared to control (n=24). FIG. 9F showsAd-Kir2.1-transduced ventricular mESC-CMs (n=15) displayed ahyperpolarized resting membrane potential relative to the controlmESC-CMs (n=24) and comparable to the adult counterparts (n=15). *denotes p<0.05.

FIGS. 10A-10E show overexpression of Kir2.1 similarly matures theelectrophysiological phenotypes of atrial mouse (m)ESC-derived CMs. FIG.10A shows a representative tracing of APs of a control atrial mESC-CMswith DAD (left) and after Ad-Kir2.1 transduction (right). Phase4-depolarization is absent in Ad-Kir2.1-transduce atrial mESC-CMs(arrow). FIG. 10B shows Ba²⁺-insensitive I_(f) and Ba²⁺-sensitive I_(K1)currents recorded from the same cells from FIG. 10A. FIG. 10C showscurrent-voltage (I-V) relationships of I_(K1) recorded from controlspontaneously firing (open circles; n=3), control quiescent (solidcircles; n=4) and Ad-Kir2.1-transduced atrial mESC-CMs (solid triangles;n=6). FIG. 10D shows Kir2.1 overexpression (n=22) rendered nearly allatrial mESC-CMs quiescent compared to control (n=29). FIG. 10E showsAd-Kir2.1-transduced atrial mESC-CMs (n=22) displayed hyperpolarizedresting membrane potential relative to control mESC-CMs (n=29), similarto their adult counterparts (n=8). * denotes p<0.05.

FIG. 11 shows representative tracings of Ca²⁺ transients in control andAd-Kir2.1-transduced mESC-CMs overexpressing Kir2.1. No significantdifferences in Ca²⁺ transients were observed.

FIGS. 12A and 12B show transmission electron microscopy images of theadditional (FIG. 12A) unconditioned ddhESC-CMs showed myofibrils thatare less dense and less organized than (FIG. 12B) the electricallyconditioned ddhESC-CMs. The z-lines (arrows) can be seen in both groups.

FIGS. 13A and 13B show electrical conditioning increases ventricularphenotype in mESC-CMs. FIG. 13A shows a representative flow cytometricdot plot of unpaced control and electrically conditioned CMs derivedfrom Lv-EF1α-GFP-MLC2v-DsRed reporter mESC line showing increasedpopulation of MLC2v positive cells in the polygonal gate. FIG. 13B showsaverage percentage of MLC2v-positive and median of unpaced control andelectrically conditioned mESC-CMs (n=5). * indicates p<0.05.

DETAILED DESCRIPTION Definitions

The practice of the present invention will employ, unless otherwiseindicated, conventional techniques of tissue culture, immunology,molecular biology, microbiology, cell biology and recombinant DNA, whichare within the skill of the art. See, e.g., Sambrook, Fritsch andManiatis, Molecular Cloning: A Laboratory Manual, 2^(nd) edition (1989);Current Protocols In Molecular Biology (F. M. Ausubel, et al. eds.,(1987)); the series Methods in Enzymology (Academic Press, Inc.): PCR 2:A Practical Approach (M. J. MacPherson, B. D. Hames and G. R. Tayloreds. (1995)); Harlow and Lane, eds. (1988) Antibodies, A LaboratoryManual; Harlow and Lane, eds. (1999) Using Antibodies, a LaboratoryManual; and Animal Cell Culture (R. I. Freshney, ed. (1987)).

All numerical designations, e.g., pH, temperature, time, concentration,and molecular weight, including ranges, are approximations which arevaried (+) or (−) by increments of 0.1. It is to be understood, althoughnot always explicitly stated that all numerical designations arepreceded by the term “about”. It also is to be understood, although notalways explicitly stated, that the reagents described herein are merelyexemplary and that equivalents of such are known in the art.

As used in the specification and claims, the singular form “a”, “an” and“the” include plural references unless the context clearly dictatesotherwise. For example, the term “a cell” includes a plurality of cells,including mixtures thereof.

“WT” is an abbreviation for “wild type.” Wild type defines the cell,composition, tissue or other biological material as it exists in nature.

The “electrophysiology” of a cell or tissue is the electrical propertiesof said cell or tissue. These electrical properties are measurements ofvoltage change or electrical current flow at variety scales including,but are not limited to, single ion channel proteins, single cells, smallpopulations of cells, tissues comprised of various cell populations, andwhole organs (e.g. the heart). Several cell types and the tissues theycomprise have electrical properties including, but not limited to,muscle cells, liver cells, pancreatic cells, ocular cells and neuronalcells. The electrical properties of a cell or tissue can be measured bythe use of electrodes (examples include, but are not limited to, simplesolid conductors including discs and needles, tracings on printedcircuit boards, and hollow tubes, such as glass pipettes, filled anelectrolyte). Intracellular recordings can be made by using techniquessuch as the voltage clamp, current clamp, patch-clamp, or sharpelectrode. Extracellular recordings can be made by using techniques suchas single unit recording, field potentials, and amperometry. A techniquefor high throughput analysis can also be used, such as the planar patchclamp. In another aspect, the Bioelectric Recognition Assay (BERA) canbe used to measure changes in the membrane potential of cells. The abovetechniques are described in the following U.S. Pat. Nos. 7,270,730;5,993,778; 6,461,860 and described in the following literature Hamill etal. (1981) Pflugers Arch. 391(2):85-100; Alvarez et al. (2002) Adv.Physiol. Educ. 26(1-4):327-341; Kornreich (2007) J. Vet. Cardiol.9(1):25-37; Perkins (2006) J. Neurosci. Methods. 154(1-2):1-18; Gurney(2000) J. Pharmacol. Toxicol. Methods. 44(22):409-420; Baker et al.(1999) J. Neurosci. Methods 94(1):5-17; McNames and Pearson (2006) ConfProc. IEEE Eng. Med. Biol. Soc. 1(1):1185-1188; Porterfield (2007)Biosens. Bioelectron. 22(7):1186-1196; Wang and Li (2003) Assay DrugDev. Technol. 1(5):695-708; and Kintzios et al. (2001) Biosens.Bioelectron. 16(4-5):325-336.

In addition to the electrophysiology of a cell or tissue being measuredby the techniques described above, the electrophysiology of largerorgans which are comprised of this cell or tissue can be measured byadditional techniques, examples of which include, but are not limited toelectrocardiograms (ECG or EKG). An ECG records the electrical activityof the heart over time. Analysis of the depolarization andrepolarization waves results a description of the electrophysiology ofthe total heart muscle. In one embodiment, an ECG can be used to measurethe cardiac function in a patient prior to and following administrationof the cells or population of cells described herein.

The term “phenotype” refers to a description of an individual's trait orcharacteristic that is measurable and that is expressed only in a subsetof individuals within a population. In one aspect of the invention, anindividual's phenotype includes the phenotype of a single cell, asubstantially homogeneous population of cells, a population ofdifferentiated cells, or a tissue comprised of a population of cells.

In one aspect, an “electrophysiological phenotype” of a cell or tissueis the measurement of a cell or tissue's action potential. An actionpotential is a spike of electrical discharge that travels along themembrane of a cell. The properties of action potentials differ dependingon the cell type or tissue. For example, cardiac action potentials aresignificantly different from the action potentials of most neuronalcells. In one embodiment, the action potential is a cardiac actionpotential. The “cardiac action potential” is a specialized actionpotential in the heart, with unique properties necessary for function ofthe electrical conduction system of the heart. The cardiac actionpotential has 5 phases; phase 4 (resting membrane potential), phase 0(rapid depolarization), phase 1 (inactivation of the fast Na⁺ channelscausing a small downward deflection of the action potential), phase 2(a.k.a. the plateau phase, is the sustained balance between inwardmovement of Ca²⁺ and outward movement of K⁺), phase 3 (cellrepolarization), and back to phase 4. The cardiac action potentials ofcells comprising the different portions of the heart have uniquefeatures and patterns specific to those cells including, atrial,ventricular, and pacemaker action potentials. One embodiment of theinvention is the electrophysiological phenotype of an adult cardiacventricular or atrial muscle cell that have normally electricallysilent-yet-excitable properties.

“I_(K1) activity” is the activity of a cell which results in the inwardrectifier current of the cell. It is contemplated that the I_(K1)activity is a stabilizer of a cell's resting membrane potential. Thisactivity is controlled by a family of proteins termed theinward-rectifier potassium ion channels (Kir channels). There are sevensubfamilies of Kir channels (Kir1, Kir2, Kir3, Kir4, Kir5, Kir6, andKir7). Each subfamily has multiple members (e.g., Kir2.1, Kir2.2,Kir2.3, etc). The Kir2 subclass has four members, Kir2.1, Kir2.2,Kir2.3, and Kir2.4. The active Kir channels are formed fromhomotetrameric membrane proteins. Additionally, heterotetramers can formbetween members of the same subfamily (e.g., Kir2.1 and Kir2.3) when thechannels are overexpressed. The proteins Kir2.1, Kir2.2, Kir2.3, andKir2.4 are also known as IRK1, IRK2, IRK3, and IRK4, respectively. Theseproteins have been sequenced and characterized, see for example GenBankAccession Nos. AAF73241, AAF73242, BAC02718, NP_000882, BAD23901, NP066292, AAL89708, P63252, P52185, P52190, O19182, O18839, Q64273,P49656, P35561, CAA56622, AAY53910, Q14500, P52188, P52187,NP_001019861, NP_690607, NP_609903, Q64198, P52189, NP_004972, AAF97619,NP_733838, Q8JZN3 and O70596. The genes for these proteins have beensequenced and characterized, see for example GenBank Accession Nos.AB074970, AF153819, NM_000891, AB182123, NM_021012, AF482710, X80417,DQ023214, NM_001024690, NM_152868, NM_004981, AF181988, and NM_170720.

“I_(f) activity” is the activity of a cell which results in the “funny”or pacemaker current of the cell. It is contemplated that this currentfunctionally modulates pacing of cells which compose the heart(specifically the cells which compose the SA node). The I_(f) activityis a mixed Na⁺/K⁺ inward current activated by hyperpolarization andmodulated by the autonomic nervous system. This activity is controlledby a family of proteins termed the hyperpolarization-activatedcyclic-nucleotide-modulated channels (HCN channels). There are fourmembers of the HCN family (e.g., HCN1, HCN2, HCN3, and HCN4). HCNisoforms have been shown to coassemble and form heteromultimers. An HCNchannel is activated by membrane hyperpolarization and modulated by cAMPand cGMP. These proteins have been sequenced and characterized, see forexample GenBank Accession Nos. AAO49470, AAO49469, NP_446136, Q9UL51,NP_001185, NP_005468, NP_065948, EDL89402, NP_445827, NP_001034410 andNP_066550. The genes for these proteins have been sequenced andcharacterized, see for example GenBank Accession Nos. AF488550,AF488549, NM_053684, NM_001194, NM_005477, NM_020897, CH474029 andNM_001039321.

Examples of electrophysiological activities include, but are not limitedto, I_(Kr), I_(Ks), I_(Na), I_(Ca), I_(tol), I_(NaCa), I_(NaK) andI_(pCa). Examples of proteins that modulate these activities includeNav1.5, Cav1.2, Kv4.2, Kv4.3, Kv7.1, Kv11.1, 3Na⁺-1Ca²⁺-exchanger(NCX1), 3Na⁺-2K⁺-ATPase, and Ca²⁺-transporting ATPase. These proteinshave been sequenced and characterized, see for example GenBank AccessionNos. CAC84530, NP_000710, CAB56841, AAF01045, NP_000209, NP_598004,CAA73478, ABV24476 and NP_001001787. The genes for these proteins havebeen sequenced and characterized, see for example GenBank Accession Nos.AJ310887, NM_000719, AJ010969, AF187964, NM_000218, NM_133497, Y13033,EU107280, NM_001001787.

A “ryanodine receptor” or (RyR) is a receptor that mediates the releaseof calcium (Ca²⁺) from the sarcoplasmic reticulum. In skeletal muscle,it is believed that activation occurs via a physical coupling to theL-type calcium channel, while in cardiac muscle, the primary mechanismis calcium-induced calcium release. There are multiple isoforms ofryanodine: RyR1 primarily expressed in skeletal muscle; RyR2 primarilyexpressed in the myocardium; RyR3 is more widely expressed, but isespecially in the brain; and a fourth form found only in fish. Theseproteins have been sequenced and characterized, see for example GenBankAccession Nos. CAA01501, AAP29981, NP 001001534, NP_001095188,NP_001076231, BAA08309, AAB29457, Q92736, AAH59061, P30957, Q15413,AAI16743, NP_996757, CAA69029, and AAB58117. The gene for this proteinhas also been sequenced and characterized, see for example GenBankAccession Nos. NM_000540, NM_001035, NM_023868, NM_001001534, NM_177652.The above GenBank Accession Nos. were last accessed on Sep. 12, 2007.

A “Sarco/Endoplasmid Reticulum Ca²⁺-ATPase” or (SERCA) is a 110-kDAtransmembrane calcium pump which transfers Ca²⁺ from the cytosol of thecell to the lumen of the sarcoplasmic reticulum at the expense of ATPhydrolysis during muscle relaxation. There are five isoforms of SERCAgenes and the cardiac/slow skeletal muscle type splicing variant of theSERCA2a gene is the predominant SERCA isotype expressed in both normaland failing hearts. These proteins have been sequenced andcharacterized, see for example GenBank Accession Nos. NP_777617,NP_777615, NP_777614, NP_004311, NP_777618, NP_777613, NP_775293,CAB38029, CAA76764, BAD73969, BAD73967, AAB82290, NP_031530, NP_058025,NP_478120, AAB08097, NP_990850, NP_001672, NP_733765, and NP_001003214.The gene for this protein has also been sequenced and characterized, seefor example GenBank Accession Nos. NM_001001396, NM_001684, NM_004320,and NM_174955. The above GenBank Accession Nos. were last accessed onSep. 12, 2007.

A “Na⁺/Ca²⁺ exchanger” or (NCX) is a transmembrane protein and member ofthe cation/Ca²⁺ antiporter family which plays a key role in maintainingcellular Ca²⁺ homeostasis in a variety of cell types. These proteinshave been sequenced and characterized, see for example GenBank AccessionNos. BAA83463, AAB50166, P70414, NP_524423, NP_732576, NP_732577,AAP37041, AAF06363, AAB46708, and NP_573484. The gene for this proteinhas also been sequenced and characterized, see for example GenBankAccession Nos. NM_170665, NM_001681, NM_001037102, NM_011406, NM_080440,NM_176632, and NM_012652. The above GenBank Accession Nos. were lastaccessed on Sep. 12, 2007.

A “cardiomyocyte or cardiac myocyte” is a specialized muscle cell whichprimarily forms the myocardium of the heart. Cardiomyocytes have fivemajor components: 1. cell membrane (sarcolemma) and T-tubules, forimpulse conduction, 2. sarcoplasmic reticulum, a calcium reservoirneeded for contraction, 3. contractile elements, 4. mitochondria, and 5.a nucleus. Stem cells can be propagated to mimic the physiologicalfunctions of cardiomyocytes or alternatively, differentiate intocardiomyocytes. This differentiation can be detected by the use markersselected from, but not limited to, myosin heavy chain, myosin lightchain, actinin, troponin, and tropomyosin.

The regulatory protein “Junctin” is a 26-kDa integral membrane protein,which forms a quaternary protein complex with the ryanodine receptor,calsequestrin and triadin at the junctional sarcoplasmic reticulummembrane in cardiac and skeletal muscles. Junctin is believed to work inconjunction with triadin as calsequestrin-anchoring proteins that couplecalsequestrin to RyR and facilitate Ca²⁺ release. These proteins havebeen sequenced and characterized, see for example GenBank Accession Nos.AAF82246, AAG16983, AAF82247, 2206415A, AAF37204, AAK00614, AAN87550,AAN87549, AAL09319, AAL09320, and AAL09321. The above GenBank AccessionNos. were last accessed on Sep. 12, 2007. The gene for this protein hasalso been sequenced and characterized, see for example Wetzel et al.(2000) Mol Genet. Metab. 69(3):252-258.

The regulatory protein “Triadin” is a 95 kDa integral membrane protein,which forms a quaternary protein complex with the ryanodine receptor(RyR), calsequestrin and junctin as described above. Triadin has alsobeen shown to interact with dihydropyridine receptors (DHPR's). Triadinhas been shown to co-localize with both DHPR and RyR at the junctionalface of the terminal cisternae. These proteins have been sequenced andcharacterized, see for example GenBank Accession Nos. NP_006064,CAC44894, Q28820, NP_001076212, NP_001003154, CAD33526, AAA75315,CAI41045, CAI19636, AAC48496, AAC48497, AAC48498, AAL33878, AAL33877,and AAL33876. The gene for this protein has also been sequenced andcharacterized, see for example GenBank Accession Nos. NM_006073,NM_001003154, and BC139910. The above GenBank Accession Nos. were lastaccessed on Sep. 12, 2007.

The protein “Calsequestrin” (“CSQ”) is the major calcium storage proteinof the SR. Intraluminar Ca²⁺ binds to calsequestrin during diastole toprevent Ca²⁺ precipitation and to lower its free ionic concentration tofacilitate efficient storage. Calsequestrin forms part of a largequaternary complex with the ryanodine receptor, triadin, and junctin asdescribed above. These proteins have been sequenced and characterized,see for example GenBank Accession Nos. NP_001222, NP_001223, BAA23494,CAI23373, CAI14532, CAI15276, EAW52736, AAH22289, AAA48674, CAA45609,NP_001095161, AAB87570, AAC69472, AAI05186, BAF34916, and AAI33410. Thegene for this protein has also been sequenced and characterized, see forexample GenBank Accession Nos. NM_001232 and NM_001231. The aboveGenBank Accession Nos. were last accessed on Sep. 12, 2007.

The protein “Phospholamban” is a 52 amino acid integral membrane proteinthat regulates the Ca²⁺ pump in cardiac muscle cells. Dephosphorylatedphospholamban interacts with the Ca²⁺ ATPase pump SERCA to lower itsactivity and sensitivity to Ca⁺², thus decreasing Ca²⁺ uptake into thesarcoplasmic reticulum. Thus, when phospholamban is phosphorylated, itinteracts with SERCA is reduced, resulting in an increase in Ca²⁺transport into the sarcoplasmic reticulum. This protein has beensequenced and characterized, see for example GenBank Accession Nos.NP_002658, NP_073198, NP_075618, NP_001003332, NP_999378, NP_001076090,NP_990741, and P61012. The gene for this protein has also been sequencedand characterized, see for example GenBank Accession Nos. NM_214213,NM_002667, NM_023129 and NM_001003332. The above GenBank Accession Nos.were last accessed on Sep. 12, 2007.

The cardiomyocyte marker “myosin heavy chain” and “myosin light chain”are part of a large family of motor proteins found in muscle cellsresponsible for producing contractile force. These proteins have beensequenced and characterized, see for example GenBank Accession Nos.AAD29948, CAC70714, CAC70712, CAA29119, P12883, NP_000248, P13533,CAA37068, ABR18779, AAA59895, AAA59891, AAA59855, AAB91993, AAH31006,NP_000423, and ABC84220. The genes for these proteins has also beensequenced and characterized, see for example GenBank Accession Nos.NM_002472 and NM_000432. The above GenBank Accession Nos. were lastaccessed on Sep. 12, 2007.

The cardiomyocyte marker “actinin” is a microfilament protein which arethe thinnest filaments of the cytoskeleton found in the cytoplasm of alleukaryotic cells. Actin polymers also play a role in actomyosin-drivencontractile processes and serve as platforms for myosin's ATPhydrolysis-dependent pulling action in muscle contraction. This proteinhas been sequenced and characterized, see for example GenBank AccessionNos. NP_001093, NP_001095, NP_001094, NP_004915, P35609, NP_598917,NP_112267, AAI07534, and NP_001029807. The gene for this protein hasalso been sequenced and characterized, see for example GenBank AccessionNos. NM_001102, NM_004924, and NM_001103. The above GenBank AccessionNos. were last accessed on Sep. 12, 2007.

The cardiomyocyte marker “troponin” is a complex of three proteins thatis integral to muscle contraction in skeletal and cardiac muscle.Troponin is attached to the protein “tropomyosin” and lies within thegroove between actin filaments in muscle tissue. Tropomyosin can be usedas a cardiomyocyte marker. These proteins have been sequenced andcharacterized, see for example GenBank Accession Nos. NP_000354,NP_003272, P19429, NP_001001430, AAB59509, AAA36771, and NP_001018007.The gene for this protein has also been sequenced and characterized, seefor example GenBank Accession Nos. NM_000363, NM_152263, andNM_001018007. The above GenBank Accession Nos. were last accessed onSep. 12, 2007.

The protein “caveolin 3” is a 151 amino acid (˜20-kDA) protein shown tobe associated with the caveolar plasma membranes and is a musclespecific form of the caveolin family. Caveolins in general are believedto act as scaffolding proteins within caveolar membranes. This proteinhas been sequenced and characterized, see for example GenBank AccessionNos. NP_203123, NP_001225, P56539, NP_062028, NP_031643, and Q2KI43. Thegene for this protein has also been sequenced and characterized, see forexample GenBank Accession Nos. NM_019155 and Z18951. The above GenBankAccession Nos. were last accessed on Sep. 12, 2007.

The protein “amphiphysin 2” (also known as Bin1) have a putative role inmembrane deformation at endocytic sites. An isoform of amphiphysin 2concentrated at T-tubules induces tubular plasma membrane invaginationswhen expressed in nonmuscle cells. In developing myotubes, amphiphysin 2and caveolin 3 segregate in tubular and vesicular portions of theT-tubules system, respectively. These proteins have been sequenced andcharacterized, see for example GenBank Accession Nos. NP_647477,CAA57197, P49418, NP_778172, and CAA73807. The gene for this protein hasalso been sequenced and characterized, see for example GenBank AccessionNos. NM_009668, NM_139343, NM_139344, NM_139346, NM_139347, NM_139349,NM_139345, NM_139348, NM_004305, NM_139350, and NM_139351. The aboveGenBank Accession Nos. were last accessed on Sep. 12, 2007.

In one aspect, an “electrophysiological phenotype” of a cell or tissueis the measurement of a cell or tissue's action potential. An actionpotential is a spike of electrical discharge that travels along themembrane of a cell. The properties of action potentials differ dependingon the cell type or tissue. For example, cardiac action potentials aresignificantly different from the action potentials of most neuronalcells. In one embodiment, the action potential is a cardiac actionpotential. The “cardiac action potential” is a specialized actionpotential in the heart, with unique properties necessary for function ofthe electrical conduction system of the heart. The cardiac actionpotential has 5 phases; phase 4 (resting membrane potential), phase 0(rapid depolarization), phase 1 (inactivation of the fast Na⁺ channelscausing a small downward deflection of the action potential), phase 2(a.k.a. the plateau phase, is the sustained balance between inwardmovement of Ca²⁺ and outward movement of K⁺), phase 3 (cellrepolarization), and back to phase 4. The cardiac action potentials ofcells comprising the different portions of the heart have uniquefeatures and patterns specific to those cells including, atrial,ventricular, and pacemaker action potentials. This action potential is aunique property of SA nodal cells and most importantly the spontaneousdepolarization (a.k.a. automaticity) necessary for SA node's pacemakeractivity. The normal activity of SA nodal cells of the heart is tospontaneously depolarize at regular rhythm, thus generating a normalheart rate. Another embodiment of the invention is theelectrophysiological phenotype of an adult cardiac ventricular or atrialmuscle cell that have normally electrically silent-yet-excitableproperties.

“I_(K1) activity” is the activity of a cell which results in the inwardrectifier current of the cell. It is contemplated that the I_(K1)activity is a stabilizer of a cell's resting membrane potential. Thisactivity is controlled by a family of proteins termed theinward-rectifier potassium ion channels (Kir channels). There are sevensubfamilies of Kir channels (Kir1, Kir2, Kir3, Kir4, Kir5, Kir6, andKir7). Each subfamily has multiple members (e.g. Kir2.1, Kir2.2, Kir2.3,etc). The Kir2 subclass has four members, Kir2.1, Kir2.2, Kir2.3, andKir2.4. The active Kir channels are formed from homotetrameric membraneproteins. Additionally, heterotetramers can form between members of thesame subfamily (e.g. Kir2.1 and Kir2.3) when the channels areoverexpressed. The proteins Kir2.1, Kir2.2, Kir2.3, and Kir2.4 are alsoknown as IRK1, IRK2, IRK3, and IRK4, respectively. These proteins havebeen sequenced and characterized, see for example GenBank Accession Nos.AAF73241, AAF73242, BAC02718, NP_000882, BAD23901, NP_066292, AAL89708,P63252, P52185, P52190, O19182, O18839, Q64273, P49656, P35561,CAA56622, AAY53910, Q14500, P52188, P52187, NP_001019861, NP_690607,NP_609903, Q64198, P52189, NP_004972, AAF97619, NP_733838, Q8JZN3 andO70596, last accessed on Sep. 28, 2007. The genes for these proteinshave been sequenced and characterized, see for example GenBank AccessionNos. AB074970, AF153819, NM_000891, AB182123, NM_021012, AF482710,X80417, DQ023214, NM_001024690, NM_152868, NM_004981, AF181988, andNM_170720, last accessed on Sep. 28, 2007.

“I_(f) activity” is the activity of a cell which results in the “funny”or pacemaker current of the cell. It is contemplated that this currentfunctionally modulates pacing of cells which compose the heart(specifically the cells which compose the SA node). The I_(f) activityis a mixed Na⁺/K⁺ inward current activated by hyperpolarization andmodulated by the autonomic nervous system.

Atrial Natriuretic Factor (ANF) is a short (approximately 28 aminoacids) peptide that is produced, stored and released by cardiac myocytesof the atria of the heart. It is released in response to atrial stretchand a variety of other signals. The amino acid and putative gene for anANF precursor is known in the art, for example at GenBank Accession No.X01471, last accessed on Sep. 10, 2008. A synthetic gene and itstranslation product ANF is reported at GenBank Accession No. X08077,last accessed on Sep. 10, 2008.

The phrase “equivalent protein” refers to protein or polynucleotidewhich hybridizes to the exemplified polynucleotide under stringentconditions and which exhibit similar or enhanced biological activity invivo, e.g., over 120%, or alternatively over 110%, or alternatively over100%, or alternatively, over 90% or alternatively over 85% oralternatively over 80%, or alternatively over 70%, as compared to thestandard or control biological activity. Additional embodiments withinthe scope of this invention are identified by having more than 80%, oralternatively, more than 85%, or alternatively, more than 90%, oralternatively, more than 95%, or alternatively more than 97%, oralternatively, more than 98 or 99% sequence identity. Percentageidentity can be determined by sequence comparison programs such as BLASTrun under appropriate conditions. In one aspect, the program is rununder default parameters.

The terms “polynucleotide” and “oligonucleotide” are usedinterchangeably and refer to a polymeric form of nucleotides of anylength, either deoxyribonucleotides or ribonucleotides or analogsthereof. Polynucleotides can have any three-dimensional structure andmay perform any function, known or unknown. The following arenon-limiting examples of polynucleotides: a gene or gene fragment (forexample, a probe, primer, EST or SAGE tag), exons, introns, messengerRNA (mRNA), transfer RNA, ribosomal RNA, ribozymes, cDNA, recombinantpolynucleotides, branched polynucleotides, plasmids, vectors, isolatedDNA of any sequence, isolated RNA of any sequence, nucleic acid probesand primers. A polynucleotide can comprise modified nucleotides, such asmethylated nucleotides and nucleotide analogs. If present, modificationsto the nucleotide structure can be imparted before or after assembly ofthe polynucleotide. The sequence of nucleotides can be interrupted bynon-nucleotide components. A polynucleotide can be further modifiedafter polymerization, such as by conjugation with a labeling component.The term also refers to both double- and single-stranded molecules.Unless otherwise specified or required, any embodiment of this inventionthat is a polynucleotide encompasses both the double-stranded form andeach of two complementary single-stranded forms known or predicted tomake up the double-stranded form.

A polynucleotide is composed of a specific sequence of four nucleotidebases: adenine (A); cytosine (C); guanine (G); thymine (T); and uracil(U) for thymine when the polynucleotide is RNA. Thus, the term“polynucleotide sequence” is the alphabetical representation of apolynucleotide molecule. This alphabetical representation can be inputinto databases in a computer having a central processing unit and usedfor bioinformatics applications such as functional genomics and homologysearching.

A “gene” refers to a polynucleotide containing at least one open readingframe (ORF) that is capable of encoding a particular polypeptide orprotein after being transcribed and translated. Any of thepolynucleotide sequences described herein may be used to identify largerfragments or full-length coding sequences of the gene with which theyare associated. Methods of isolating larger fragment sequences are knownto those of skill in the art.

The term “express” refers to the production of a gene product.

As used herein, “expression” refers to the process by whichpolynucleotides are transcribed into mRNA and/or the process by whichthe transcribed mRNA is subsequently being translated into peptides,polypeptides, or proteins. If the polynucleotide is derived from genomicDNA, expression may include splicing of the mRNA in a eukaryotic cell.“Differentially expressed” as applied to a gene, refers to thedifferential production of the mRNA transcribed from the gene or theprotein product encoded by the gene. A differentially expressed gene maybe overexpressed or underexpressed (a.k.a. inhibited) as compared to theexpression level of a normal or control cell. In one aspect, it refersto overexpression that is 1.5 times, or alternatively, 2 times, oralternatively, at least 2.5 times, or alternatively, at least 3.0 times,or alternatively, at least 3.5 times, or alternatively, at least 4.0times, or alternatively, at least 5 times, or alternatively 10 timeshigher (i.e., and therefore overexpressed) or lower than the expressionlevel detected in a control sample. The term “differentially expressed”also refers to nucleotide sequences in a cell or tissue which areexpressed where silent in a control cell or not expressed whereexpressed in a control cell.

A “gene product” or alternatively a “gene expression product” refers tothe amino acid (e.g., peptide or polypeptide) generated when a gene istranscribed and translated.

“Connexin” or gap junction proteins, are a family of structurallyrelated transmembrane proteins that assemble to form vertebrate gapjunctions. Each gap junction comprises 2 hemichannels, or “connexons”,which are themselves each constructed out of 6 connexin proteins. It iscontemplated that these gap junctions are essential for propercoordinated depolarization of cardiomyocytes composing heart muscle.Connexins are most commonly named according to their molecular weights(e.g. Cx26 is the connexin protein of 26 kDa). However, these proteinsare also known by a different nomenclature known as the Gja/Gjb system.A description of this system can be found at(http://www.gene.ucl.ac.uk/nomenclature/genefamily/gj.html). Theseproteins have been sequenced and characterized, see for example GenBankAccession Nos. NP_003995, Q6PEY0, NP_940970, NP_006774, NP_853516,NP_699199, NP_689343, NP_694944, 075712, NP_001005752, NP_005259,P08034, NP_000157, NP_065711, NP_000156, NP_005488, NP_068773, Q5T442,AAH89439, NP_005258, NP_110399 and EAW48537. The genes for theseproteins have been sequenced and characterized, see for example GenBankAccession Nos. NM_004004, NM_198568, NM_006783, NM_181538, NM_153368,NM_152219, NM_153212, NM_005268, NM_000166, NM_020660, NM_000165,NM_005497, NM_021954, BC089439, NM_005267, NM_030772, and CH471051.

“Under transcriptional control” is a term well understood in the art andindicates that transcription of a polynucleotide sequence, usually a DNAsequence, depends on its being operatively linked to an element whichcontributes to the initiation of, or promotes, transcription.“Operatively linked” intends the polynucleotides are arranged in amanner that allows them to function in a cell.

“Gene delivery,” “gene transfer,” “transducing,” and the like as usedherein, are terms referring to the introduction of an exogenouspolynucleotide (sometimes referred to as a “transgene”) into a hostcell, irrespective of the method used for the introduction. Such methodsinclude a variety of well-known techniques such as vector-mediated genetransfer (by, e.g., viral infection/transfection, or various otherprotein-based or lipid-based gene delivery complexes) as well astechniques facilitating the delivery of “naked” polynucleotides (such aselectroporation, “gene gun” delivery and various other techniques usedfor the introduction of polynucleotides). The introduced polynucleotidemay be stably or transiently maintained in the host cell. Stablemaintenance typically requires that the introduced polynucleotide eithercontains an origin of replication compatible with the host cell orintegrates into a replicon of the host cell such as an extrachromosomalreplicon (e.g., a plasmid) or a nuclear or mitochondrial chromosome. Anumber of vectors, e.g., viral vectors, are known to be capable ofmediating transfer of genes to mammalian cells, as is known in the artand described herein.

A “probe” when used in the context of polynucleotide manipulation refersto an oligonucleotide that is provided as a reagent to detect a targetpotentially present in a sample of interest by hybridizing with thetarget. Usually, a probe will comprise a label or a means by which alabel can be attached, either before or subsequent to the hybridizationreaction. Suitable labels include, but are not limited to radioisotopes,fluorochromes, chemiluminescent compounds, dyes, and proteins, includingenzymes.

A “primer” is a short polynucleotide, generally with a free 3′ —OH groupthat binds to a target or “template” potentially present in a sample ofinterest by hybridizing with the target, and thereafter promotingpolymerization of a polynucleotide complementary to the target. A“polymerase chain reaction” (“PCR”) is a reaction in which replicatecopies are made of a target polynucleotide using a “pair of primers” ora “set of primers” consisting of an “upstream” and a “downstream”primer, and a catalyst of polymerization, such as a DNA polymerase, andtypically a thermally-stable polymerase enzyme. Methods for PCR are wellknown in the art, and taught, for example in “PCR: A Practical Approach”(M. MacPherson et al., IRL Press at Oxford University Press (1991)). Allprocesses of producing replicate copies of a polynucleotide, such as PCRor gene cloning, are collectively referred to herein as “replication.” Aprimer can also be used as a probe in hybridization reactions, such asSouthern or Northern blot analyses. Sambrook et al., infra.

“Hybridization” refers to a reaction in which one or morepolynucleotides react to form a complex that is stabilized via hydrogenbonding between the bases of the nucleotide residues. The hydrogenbonding may occur by Watson-Crick base pairing, Hoogstein binding, or inany other sequence-specific manner. The complex may comprise two strandsforming a duplex structure, three or more strands forming amulti-stranded complex, a single self-hybridizing strand, or anycombination of these. A hybridization reaction may constitute a step ina more extensive process, such as the initiation of a PCR reaction, orthe enzymatic cleavage of a polynucleotide by a ribozyme.

Hybridization reactions can be performed under conditions of different“stringency”. In general, a low stringency hybridization reaction iscarried out at about 40° C. in 10×SSC or a solution of equivalent ionicstrength/temperature. A moderate stringency hybridization is typicallyperformed at about 50° C. in 6×SSC, and a high stringency hybridizationreaction is generally performed at about 60° C. in 1×SSC.

When hybridization occurs in an antiparallel configuration between twosingle-stranded polynucleotides, the reaction is called “annealing” andthose polynucleotides are described as “complementary”. Adouble-stranded polynucleotide can be “complementary” or “homologous” toanother polynucleotide, if hybridization can occur between one of thestrands of the first polynucleotide and the second. “Complementarity” or“homology” (the degree that one polynucleotide is complementary withanother) is quantifiable in terms of the proportion of bases in opposingstrands that are expected to form hydrogen bonding with each other,according to generally accepted base-pairing rules.

A polynucleotide or polynucleotide region (or a polypeptide orpolypeptide region) has a certain percentage (for example, 80%, 85%,90%, or 95%) of “sequence identity” to another sequence means that, whenaligned, that percentage of bases (or amino acids) are the same incomparing the two sequences. This alignment and the percent homology orsequence identity can be determined using software programs known in theart, for example those described in Current Protocols in MolecularBiology (F. M. Ausubel et al., eds., 1987) Supplement 30, section7.7.18, Table 7.7.1. Preferably, default parameters are used foralignment. A preferred alignment program is BLAST, using defaultparameters. In particular, preferred programs are BLASTN and BLASTP,using the following default parameters: Genetic code=standard;filter=none; strand=both; cutoff=60; expect=10; Matrix=BLOSUM62;Descriptions=50 sequences; sort by=HIGH SCORE; Databases=non-redundant,GenBank+EMBL+DDBJ+PDB+GenBank CDStranslations+SwissProtein+SPupdate+PIR. Details of these programs can befound at the following Internet address:http://www.ncbi.nlm.nih.gov/cgi-bin/BLAST.

The term “polypeptide” is used interchangeably with the term “protein”and in its broadest sense refers to a compound of two or more subunitamino acids, amino acid analogs or peptidomimetics. The subunits may belinked by peptide bonds. In another embodiment, the subunit may belinked by other bonds, e.g., ester, ether, etc. As used herein the term“amino acid” refers to either natural and/or unnatural or syntheticamino acids, including glycine and both the D and L optical isomers,amino acid analogs and peptidomimetics. A peptide of three or more aminoacids is commonly called an oligopeptide if the peptide chain is short.If the peptide chain is long, the peptide is commonly called apolypeptide or a protein.

“Under transcriptional control” is a term well understood in the art andindicates that transcription of a polynucleotide sequence, usually a DNAsequence, depends on its being operatively linked to an element whichcontributes to the initiation of, or promotes, transcription.“Operatively linked” refers to a juxtaposition wherein the elements arein an arrangement allowing them to function.

As used herein, the term “comprising” is intended to mean that thecompositions and methods include the recited elements, but not excludingothers. “Consisting essentially of” when used to define compositions andmethods, shall mean excluding other elements of any essentialsignificance to the combination for the stated purpose. Thus, acomposition consisting essentially of the elements as defined hereinwould not exclude trace contaminants from the isolation and purificationmethod and pharmaceutically acceptable carriers, such as phosphatebuffered saline, preservatives and the like. “Consisting of” shall meanexcluding more than trace elements of other ingredients and substantialmethod steps for administering the compositions of this invention orprocess steps to produce a composition or achieve an intended result.Embodiments defined by each of these transition terms are within thescope of this invention.

The term “isolated” means separated from constituents, cellular andotherwise, in which the cell, tissue, polynucleotide, peptide,polypeptide, protein, antibody or fragment(s) thereof, which arenormally associated in nature. For example, an isolated polynucleotideis separated from the 3′ and 5′ contiguous nucleotides with which it isnormally associated in its native or natural environment, e.g., on thechromosome. As is apparent to those of skill in the art, a non-naturallyoccurring polynucleotide, peptide, polypeptide, protein, antibody orfragment(s) thereof, does not require “isolation” to distinguish it fromits naturally occurring counterpart. An isolated cell is a cell that isseparated form tissue or cells of dissimilar phenotype or genotype.

As used herein, “stem cell” defines a cell with the ability to dividefor indefinite periods in culture and give rise to specialized cells. Atthis time and for convenience, stem cells are categorized as somatic(adult) or embryonic. A somatic stem cell is an undifferentiated cellfound in a differentiated tissue that can renew itself (clonal) and(with certain limitations) differentiate to yield all the specializedcell types of the tissue from which it originated. An embryonic stemcell is a primitive (undifferentiated) cell from the embryo that has thepotential to become a wide variety of specialized cell types. Anembryonic stem cell is one that has been cultured under in vitroconditions that allow proliferation without differentiation for monthsto years. Pluripotent embryonic stem cells can be distinguished fromother types of cells by the use of marker including, but not limited to,Oct-4, alkaline phosphatase, CD30, TDGF-1, GCTM-2, Genesis, Germ cellnuclear factor, SSEA1, SSEA3, and SSEA4. The term “stem cell” alsoincludes “dedifferentiated” stem cells, an example of which is a somaticcell which is directly converted to a stem cell, i.e. reprogrammed. Aclone is a line of cells that is genetically identical to theoriginating cell; in this case, a stem cell.

An “induced pluripotent cell” intends embryonic-like cells reprogrammedto the immature phenotype from adult cells. Various methods are known inthe art, e.g., “A simple new way to induce pluripotency” Nature, 29 Jan.2014 and available at sciencedaily.com/releases/2014/01/140129184445,last accessed on Feb. 5, 2014 and U.S. Patent Application PublicationNo. 2010/0041054. Human iPSCs also express stem cell markers and arecapable of generating cells characteristic of all three germ layers.

The term “propagate” means to grow or alter the phenotype of a cell orpopulation of cells. The term “growing” or “expanding” refers to theproliferation of cells in the presence of supporting media, nutrients,growth factors, support cells, or any chemical or biological compoundnecessary for obtaining the desired number of cells or cell type. In oneembodiment, the growing of cells results in the regeneration of tissue.In yet another embodiment, the tissue is comprised of cardiomyocytes.

The term “culturing” refers to the in vitro propagation of cells ororganisms on or in media of various kinds. It is understood that thedescendants of a cell grown in culture may not be completely identical(i.e., morphologically, genetically, or phenotypically) to the parentcell. By “expanded” is meant any proliferation or division of cells.

“Clonal proliferation” refers to the growth of a population of cells bythe continuous division of single cells into two identical daughtercells and/or population of identical cells.

As used herein, the “lineage” of a cell defines the heredity of thecell, i.e., its predecessors and progeny. The lineage of a cell placesthe cell within a hereditary scheme of development and differentiation.

“Differentiation” describes the process whereby an unspecialized cellacquires the features of a specialized cell such as a heart, liver, ormuscle cell. “Directed differentiation” refers to the manipulation ofstem cell culture conditions to induce differentiation into a particularcell type or phenotype. “Dedifferentiated” defines a cell that revertsto a less committed position within the lineage of a cell. As usedherein, the term “differentiates or differentiated” defines a cell thattakes on a more committed (“differentiated”) position within the lineageof a cell. As used herein, “a cell that differentiates into a mesodermal(or ectodermal or endodermal) lineage” defines a cell that becomescommitted to a specific mesodermal, ectodermal or endodermal lineage,respectively. Examples of cells that differentiate into a mesodermallineage or give rise to specific mesodermal cells include, but are notlimited to, cells that are adipogenic, leiomyogenic, chondrogenic,cardiogenic, dermatogenic, hematopoetic, hemangiogenic, myogenic,nephrogenic, urogenitogenic, osteogenic, pericardiogenic, or stromal.

Examples of cells that differentiate into ectodermal lineage include,but are not limited to epidermal cells, neurogenic cells, andneurogliagenic cells.

Examples of cells that differentiate into endodermal lineage include,but are not limited to pleurogenic cells, and hepatogenic cells, cellthat give rise to the lining of the intestine, and cells that give riseto pancreogenic and splanchogenic cells.

As used herein, a “pluripotent cell” defines a less differentiated cellthat can give rise to at least two distinct (genotypically and/orphenotypically) further differentiated progeny cells.

A “multi-lineage stem cell” or “multipotent stem cell” refers to a stemcell that reproduces itself and at least two further differentiatedprogeny cells from distinct developmental lineages. The lineages can befrom the same germ layer (i.e., mesoderm, ectoderm or endoderm), or fromdifferent germ layers. An example of two progeny cells with distinctdevelopmental lineages from differentiation of a multilineage stem cellis a myogenic cell and an adipogenic cell (both are of mesodermalorigin, yet give rise to different tissues). Another example is aneurogenic cell (of ectodermal origin) and adipogenic cell (ofmesodermal origin).

As used herein, the term “electrical pacing” intends to apply a smallelectrical current(s) at particular frequencies and durations toartificially produce or mimic a cardiac contraction (e.g., a heartbeat).

“Substantially homogeneous” describes a population of cells in whichmore than about 50%, or alternatively more than about 60%, oralternatively more than 70%, or alternatively more than 75%, oralternatively more than 80%, or alternatively more than 85%, oralternatively more than 90%, or alternatively, more than 95%, of thecells are of the same or similar phenotype. Phenotype can be determinedby a pre-selected cell surface marker or other marker, e.g., myosin oractin or the expression of a gene or protein,

A “biocompatible scaffold” refers to a scaffold or matrix fortissue-engineering purposes with the ability to perform as a substratethat will support the appropriate cellular activity to generate thedesired tissue, including the facilitation of molecular and mechanicalsignaling systems, without eliciting any undesirable effect in thosecells or inducing any undesirable local or systemic responses in theeventual host. In other embodiments, a biocompatible scaffold is aprecursor to an implantable device which has the ability to perform itsintended function, with the desired degree of incorporation in the host,without eliciting an undesirable local or systemic effects in the host.Biocompatible scaffolds are described in U.S. Pat. No. 6,638,369.

A “composition” is intended to mean a combination of active agent, cellor population of cells and another compound or composition, inert (forexample, a detectable agent or label) or active, such as a biocompatiblescaffold.

A “pharmaceutical composition” is intended to include the combination ofan active agent with a carrier, inert or active such as a biocompatiblescaffold, making the composition suitable for diagnostic or therapeuticuse in vitro, in vivo or ex vivo.

As used herein, the term “pharmaceutically acceptable carrier”encompasses any of the standard pharmaceutical carriers, such as aphosphate buffered saline solution, water, and emulsions, such as anoil/water or water/oil emulsion, and various types of wetting agents.The compositions also can include stabilizers and preservatives. Forexamples of carriers, stabilizers and adjuvants, see Martin, Remington'sPharm. Sci., 15th Ed. (Mack Publ. Co., Easton (1975)).

An “effective amount” is an amount sufficient to effect beneficial ordesired results. An effective amount can be administered in one or moreadministrations, applications or dosages.

A “subject,” “individual” or “patient” is used interchangeably herein,and refers to a vertebrate, preferably a mammal, more preferably ahuman. Mammals include, but are not limited to, murines, rats, simians,bovines, canines, humans, farm animals, sport animals and pets.

Unmodified cells are sometimes referred to as “source cells” or “sourcestem cells”. The cells may be prokaryotic or eukaryotic, and include butare not limited to bacterial cells, yeast cells, plant cells, insectcells, animal cells, and mammalian cells, e.g., murines, rats, simians,bovines, canines, porcines and humans.

In one embodiment, an “immature cell” refers to a cell which does notpossess the desired (adult) phenotype or genotype. For example, in oneembodiment, a mature cell is a cell that is being replaced. The immaturecell can be subjected to techniques including physical, biological, orchemical processes which changes, initiates a change, or alters thephenotype or genotype of the cell into a “mature cell.” A “mature cell”refers to a cell which possess the desired phenotype or genotype. In oneembodiment, a mature cell has the phenotype or genotype of, but is notlimited to, an adult cardiomyocyte, atrial cardiomyocyte, ventricularcardiomyocyte.

A “control” is an alternative subject or sample used in an experimentfor comparison purpose. A control can be “positive” or “negative”. Forexample, where the purpose of the experiment is to determine acorrelation of an altered expression level of a gene with a particularphenotype, it is generally preferable to use a positive control (asample from a subject, carrying such alteration and exhibiting thedesired phenotype), and a negative control (a subject or a sample from asubject lacking the altered expression or phenotype).

As used herein, the terms “treating,” “treatment” and the like are usedherein to mean obtaining a desired pharmacologic and/or physiologiceffect. The effect can be prophylactic in terms of completely orpartially preventing a disorder or sign or symptom thereof, and/or canbe therapeutic in terms of a partial or complete cure for a disorderand/or adverse effect attributable to the disorder. Examples of“treatment” include but are not limited to: preventing a disorder fromoccurring in a subject that may be predisposed to a disorder, but hasnot yet been diagnosed as having it; inhibiting a disorder, i.e.,arresting its development; and/or relieving or ameliorating the symptomsof disorder, e.g., cardiac arrhythmia. As is understood by those skilledin the art, “treatment” can include systemic amelioration of thesymptoms associated with the pathology and/or a delay in onset ofsymptoms such as chest pain. Clinical and sub-clinical evidence of“treatment” will vary with the pathology, the individual and thetreatment.

The phrase “cardiac malfunction” refers to the heart, portions of theheart, or individual cells of the heart which do not have the properelectrophysiological phenotype to perform their necessary activity tomaintain normal beating of the heart muscle. Cardiac malfunction can becaused by, but not limited to, diseases or disorders including sicksinus syndrome, congestive heart failure, isolated diastolic heartfailure, bradyarrhythmia, atrial tachyarrhythmia, ventriculartachyarrhythmia, myocardial infarction, and cardiac arrhythmia. Cardiacarrhythmia includes, but is not limited to, bradycardia, tachycardia,abnormal sinus node function, or atrioventricular block.

Modified Cells and Populations of Cells

One embodiment of the invention is an isolated electrophysiologicallyimmature cell that has been modified by electrical pacing to provide thephenotype of an electrophysiologically mature cell. Examples of cellsthat can be modified include, but are not limited to embryonic stemcells, iPSCs, progenitor cells and adult stem cells that possess theability to further differentiate into cells of a desired lineage. Thecells can be isolated from a host or can be obtained from an establishedcell culture. Methods to isolate and culture ESC are known in the artand described in Xue et al. (2005) Circulation 111:11-20, Thomson et al.(1998) Science 282:1145-1147, Moore et al. (2005) ReproductiveToxicology 20:377-391, and Wang et al. (2005) Stem Cells 23:1526-1534.Available sources of these cells include, for example, from the NIHHuman Embryonic Stem Cell Registry.

The cells can be from any suitable source, e.g., an animal orvertebrate. Non-limiting examples include murine, rat, canine, simian,porcine and human.

The mature electrophysiology phenotype is obtained by modifying thephenotype of the source cell. In one aspect, the modification of thesource cell or its derivative expressly excludes transducing the sourcecell with a polynucleotide that modulates I_(K1) and I_(f) activity ofthe cell. In another aspect, modification of the cell or its derivativeexpressly excludes modification by transducing the cell with apolynucleotide that promotes or inhibits the expression of a proteinthat modulates I_(K1) activity. Examples of proteins that modulateI_(K1) and CSQ activity of the cell include, but are not limited to, theKir2 family of proteins and the HCN family of proteins. The Kir2 familyincludes the Kir2.1, Kir2.2, Kir2.3, Kir2.4 and a functionallyequivalent protein thereof. In yet another aspect, the modificationexpressly excludes modification of the isolated electrophysiologicallyimmature cell to comprise a polynucleotide that encodes a Connexinprotein or a polypeptide that enhances the expression of a Connexinprotein. Examples of Connexin proteins include, but are not limited toCx23, Cx25, Cx26, Cx30.2, Cx30, Cx31.9, Cx30.3, Cx31, Cx31.1, Cx32,Cx36, Cx43, Cx45, Cx46, Cx47, Cx50, Cx59, and Cx62.

Another embodiment of the disclosure is an isolatedelectrophysiologically immature cell that has other modifiedelectrophysiology activities including, but not limited to, I_(Kr),I_(Ks), I_(Na), I_(Ca), I_(tol), I_(NaCa), I_(NaK) and I_(pCa) activity.Examples of proteins that modulate these activities include Nav1.5,Cav1.2, Kv4.2, Kv4.3, Kv7.1, Kv11.1, 3Na⁺-1Ca²⁺-exchanger,3Na⁺-2K⁺-ATPase, and Ca²⁺-transporting ATPase.

This invention also provides a cell that has been modified as describedabove, wherein the cell further expresses a cardiomyocyte markerselected from, but not limited to, myosin heavy chain, myosin lightchain, actinin, troponin, tropomyosin and cardiac calcium handlingproteins. Examples of calcium channel proteins include, but are notlimited to ryanodine receptor (RyR), calsequestrin, junctin, triadin,phospholamban. In one aspect, the cell expresses on or more calcium pumpproteins, sarco/endoplasmic reticulum CA²⁺-ATPase (SERCA) protein,and/or reduced expression of the Na⁺/Ca²⁺ exchanger (NCX) protein.

The immature or source cell is modified to provide the desiredelectrophysiological phenotype by contacting the cell with an electricalcurrent, e.g., by electrical pacing of the cells.

This invention also provides a substantially homogeneous population ofelectrophysiologically immature cells that have been modified asdescribed above. One embodiment of the invention is a substantiallyhomogeneous population of electrophysiological immature cells that aremodified by electrical pacing.

Another embodiment of the invention is a substantially homogeneouspopulation of electrophysiologically immature cells, in one aspect aclonal population, that have other modified electrophysiology activitiesincluding, but not limited to, I_(Kr), I_(Ks), I_(Na), I_(Ca), I_(tol),I_(NaCa), I_(NaK) and I_(pCa) activity. Examples of proteins thatmodulate these activities include Nav1.5, Cav1.2, Kv4.2, Kv4.3, Kv7.1,Kv11.1, 3Na⁺-1Ca²⁺-exchanger, 3Na⁺-2K⁺-ATPase, and Ca²⁺-transportingATPase. This invention also provides a substantially homogeneouspopulation of electrophysiologically immature cells that has beenmodified as described above, wherein the cells further express acardiomyocyte marker selected from, but not limited to, myosin heavychain, myosin light chain, actinin, troponin and tropomyosin. Examplesof proteins that modulate these activities include Nav1.5, Cav1.2,Kv4.2, Kv4.3, Kv7.1, Kv11.1, 3Na⁺-1Ca²⁺-exchanger, 3Na⁺-2K⁺-ATPase, andCa²⁺-transporting ATPase. In one aspect, the population the cellsexpress cardiac calcium handling proteins. Examples of calcium channelproteins include, but are not limited to ryanodine receptor (RyR),calsequestrin, junctin, triadin, phospholamban. In one aspect, the cellexpresses on or more calcium pump proteins, sarco/endoplasmic reticulumCA²⁺-ATPase (SERCA) protein, and/or reduced expression of the Na⁺/Ca²⁺exchanger (NCX) protein in the cell.

In one aspect of the disclosure including all of the above embodiments,the source cells are comprised of embryonic stem cells, iPSCs orpluripotent stem cells. In another aspect of the disclosure includingall of the above embodiments, the substantially homogeneous populationof cells are comprised of mammalian cells. In a further embodiment, themammalian cells are human cells. Compositions and methods todifferentiate stem cells to cardiac cells are known in the art, e.g.,U.S. Pat. No. 6,387,369 and U.S. Patent Application Publication No.2007/0025972 A1.

This disclosure also provides a population of cells that have beendifferentiated from electrophysiologically immature cells and modifiedas described above, wherein the cells further express a cardiomyocytemarker selected from, but not limited to, myosin heavy chain, myosinlight chain, actinin, troponin and tropomyosin, and/or a cardiac calciumhandling protein. In another aspect, the cardiomyocytes may be furtherdifferentiated into atrial cardiomyocytes or ventricular cardiomyocytes.In another aspect of the invention, including all of the aboveembodiments, the population of cells that have been differentiated fromelectrophysiologically immature cells are comprised of embryonic stemcells or pluripotent stem cells. In another aspect of the inventionincluding all of the above embodiments, the population of cells thathave been differentiated from electrophysiologically immature cells arecomprised of mammalian cells. In a further embodiment, the mammaliancells are human cells. Compositions and methods to differentiate stemcells to cardiac cells are known in the art, e.g., U.S. Pat. No.6,387,369 and U.S. Patent Application Publication No. 2007/0025972 A1.

Yet another embodiment of the invention is a composition of any one ofthe above-noted independent modifications and a carrier. In anotherfurther embodiment, the carrier is, but not limited to, a biocompatiblescaffold, or a pharmaceutically acceptable carrier.

Further provided by this invention are any one or more combinations ofthe above-noted independent modifications. Thus, Applicants' inventionincludes any one or more combination of the independently describedmodifications. The preferred modification or combination ofmodifications will be determined by the use of the modified cells and insome aspects, the patient to be treated with the modified cell orpopulation of cells.

Also provided by this invention is a population of differentiated cellsproduced by propagating the above-noted isolated cell(s) orsubstantially homogeneous population of cells. In one aspect, the cellsand/or populations are propagated under conditions that produce immatureor mature cardiomyocytes, atrial cells or ventricular cells. Thesemethods are known to those skilled in the art and are described, forexample in Xue et al. (2005) Circulation 111:11-20, Moore et al. (2005)Reproductive Toxicology 20:377-391, and Wang et al. (2005) Stem Cells23:1526-1534.

In a separate aspect, they are propagated under conditions that produceclonal populations of substantially identical or identical cells.

Methods to Produce Modified Cells and Populations of Cells

In one aspect, the disclosure provides a method for inducing expressionof the Kir2.1 gene in a cell lacking functional Kir2.1 gene expression,with the proviso that the method excludes genetic manipulation of thecell, comprising, or alternatively consisting essentially of, or yetfurther consisting of, administering to the cell an effective amount ofelectrical pacing, thereby inducing expression of Kir2.1 gene expressionin the cell.

In another aspect, a method is provided for inducing anelectrophysiologically mature phenotype in a cell that is not terminallydifferentiated, comprising, or alternatively consisting essentially of,or yet further consisting of, administering to the cell that is notterminally differentiated an effective amount of electrical pacing,thereby inducing an electrophysiologically mature phenotype in the cell.In one aspect, the method excludes genetic manipulation of the cell toenhance or increase Kir2.1 expression.

In another aspect, a method for preparing an electrophysiologicallymature cardiomyocyte from a cell that is not terminally differentiated,comprising, or alternatively consisting essentially of, or yet furtherconsisting of, administering to the cell that is not terminallydifferentiated, an effective amount of electrical pacing, therebypreparing an electrophysiologically mature cardiomyocyte. In one aspect,the method excludes genetic manipulation of the cell to enhance orincrease Kir2.1 expression and other cardiac ion channels, e.g., the Cahandling proteins.

Cells that can be used in the method include cells that are notterminally differentiated, e.g., an embryonic stem cell, a pluripotentstem cell, an embryoid body, a mesodermal cardiosphere or an inducedpluripotent stem cell.

In one aspect, the electrophysiologically mature cardiomyocyte exhibitsone or more of a decrease of the atrial natriuretic factor (ANF) and/oran increase in one or more of the contractile proteins MHCα, MHCβ,MLC2a, MLC2v, cardiac calcium handling proteins.

In another aspect, the electrophysiologically mature cardiomyocytecomprises a cardiac cell of the group of a ventricular cell, an atrialcell. The cell is a mammalian cell, e.g., a murine cell, a bovine cell,a canine cell, a feline cell, an equine cell or a human cell.

In any of the above methods, the electrical pacing comprises a pulsedelectrical current of about 2.5 v/cm, for about 5 msecs at a width of 1HZ. In another aspect, the electrical pacing comprises a pulsedelectrical current of about 1, 2.5, 10, etc. v/cm, for about 2, 5, 10msecs, etc. at a frequency of 1, 2, 3, 10 HZ etc for various durations(1, 3, 5, 7 days, 2, 3, 4 weeks, etc.).

Further provided is an isolated cell prepared by the method as describedabove herein. In one aspect, the cell can be expanded or cultured toproduce a population of cells that in one aspect, are substantiallyhomogenous. In a further aspect, the substantially homogenous populationis a clonal population. After the cell has been modified by a method asdisclosed herein, it may be expanded to a substantially homogenouspopulation (e.g., a clonal population) of these cells or alternatively,differentiated to a more mature cell type. Compositions containing thesecells and populations of cells are also provided by this disclosure.

Thus, in one aspect, this invention provides an isolatedelectrophysiologically immature cell, such as an embryonic stem cell orpluripotent stem cell or their derivatives, that has been modified toprovide a mature electrophysiological phenotype. The invention alsoprovides for a clonal population or a population of cells differentiatedfrom electrophysiologically immature cells to provide a matureelectrophysiological phenotype. In one aspect, the cells or tissues arefurther characterized by not being modified in one or more of thefollowing manners: by transduction of a polynucleotide that promotes orinhibits I_(K1) activity of the cells; by transduction of apolynucleotide that modulates Kir2 and CSQ protein expression; bytransduction of a polynucleotide that encodes a Connexin protein orenhances the expression of a Connexin protein; and by transduction of apolynucleotide that modifies other critical electrophysiologicalactivities of the cells such as I_(Kr), I_(Ks), I_(Na), I_(Ca), I_(tol),I_(NaCa), I_(NaK) and I_(pCa).

Detection

One can determine if the cell is a more mature cell by using methodsknown in the art, e.g., by traditional hybridization techniques,immunohistochemistry or PCR. Specific examples include hybridization toDNA microarrays, in situ hybridization, PCR, RNase protection assays andNorthern blot analysis. Alternatively expression of the encodedpolypeptide can be detected using antibodies that specifically recognizeand bind the polypeptide or protein. Such antibodies are useful forvisualizing cells that express the polypeptide using techniques such asimmunohistology, ELISA, and Western blotting.

As used herein, an “antibody” includes whole antibodies and any antigenbinding fragment or a single chain thereof. Thus the term “antibody”includes any protein or peptide containing molecule that comprises atleast a portion of an immunoglobulin molecule. Examples of such include,but are not limited to a complementarity determining region (CDR) of aheavy or light chain or a ligand binding portion thereof, a heavy chainor light chain variable region, a heavy chain or light chain constantregion, a framework (FR) region, or any portion thereof, or at least oneportion of a binding protein, any of which can be incorporated into anantibody of the present invention. The term “antibody” is furtherintended to encompass digestion fragments, specified portions,derivatives and variants thereof, including antibody mimetics orcomprising portions of antibodies that mimic the structure and/orfunction of an antibody or specified fragment or portion thereof,including single chain antibodies and fragments thereof. Examples ofbinding fragments encompassed within the term “antigen binding portion”of an antibody include a Fab fragment, a monovalent fragment consistingof the V_(L), V_(H), C_(L) and CH, domains; a F(ab′)2 fragment, abivalent fragment comprising two Fab fragments linked by a disulfidebridge at the hinge region; a Fd fragment consisting of the V_(H) andC_(H), domains; a Fv fragment consisting of the V_(L) and V_(H) domainsof a single arm of an antibody, a dAb fragment (Ward et al. (1989)Nature 341:544-546), which consists of a V_(H) domain; and an isolatedcomplementarity determining region (CDR). Furthermore, although the twodomains of the Fv fragment, V_(L) and V_(H), are coded for by separategenes, they can be joined, using recombinant methods, by a syntheticlinker that enables them to be made as a single protein chain in whichthe V_(L) and V_(H) regions pair to form monovalent molecules (known assingle chain Fv (scFv)). Bird et al. (1988) Science 242:423-426 andHuston et al. (1988) Proc. Natl. Acad Sci. USA 85:5879-5883. Singlechain antibodies are also intended to be encompassed within the term“fragment of an antibody.” Any of the above-noted antibody fragments areobtained using conventional techniques known to those of skill in theart, and the fragments are screened for binding specificity andneutralization activity in the same manner as are intact antibodies.

Various antibody preparations can also be used in analytical methodssuch as ELISA assays or Western blots to demonstrate the expression ofproteins encoded by the identified genes by test cells in vitro or invivo. Fragments of such proteins generated by protease degradationduring metabolism can also be identified by using appropriate polyclonalantisera with samples derived from experimental samples.

Compositions

This invention also provides compositions containing the cells,population of cells and/or differentiated cells in combination with acarrier, such as a biocompatible scaffold or a pharmaceuticallyacceptable carrier. In one embodiment, the composition is intended fortherapeutic use and therefore, an effective amount of the modified cell,population of cells or differentiated cells are provided in thecomposition.

Uses of the Cells and Cell Populations

Yet another embodiment of the invention is a method for restoringcardiac function in a tissue or host in need thereof. This and othertherapeutic uses are described herein.

In one embodiment, the invention provides methods for regeneratingcardiac muscle tissue by growing an effective amount of the modifiedcell or population of immature cells described above. Yet anotherembodiment of the invention is a method for regenerating cardiac muscletissue by growing an effective amount of a substantially homogeneouspopulation of immature cells described above.

Yet another embodiment of the invention is a method for regeneratingcardiac muscle tissue in a suitable host by administering to the host aneffective amount of the isolated cell or population of cells asdescribed above.

A further embodiment of the invention is the host is a mammalian patientand the isolated cell is mammalian. In another embodiment, the host is ahuman patient and the isolated cell is human.

Another embodiment of the invention is a method for regenerating cardiacmuscle tissue in a suitable host by administering to the host aneffective amount of an isolated electrically paced cell as describedherein. In a further embodiment the host is a mammalian patient and theisolated cell is mammalian. In another embodiment, the host is a humanpatient and the isolated cell is human.

Another embodiment of the invention is a method of improving cardiacfunction in a patient in need thereof by the administration of aneffective amount of an electrically paced cell as described herein toprovide the phenotype of an electrophysiologically mature cell. Thepatients of this embodiment are suffering from a disease or disorderassociated with cardiac malfunction including, but not limited to,congestive heart failure, isolated diastolic heart failure, myocardialinfarction, and cardiac arrhythmia. There are several forms of cardiacarrhythmia that can be treated including, but not limited to, sick sinussyndrome, bradyarrhythmia, abnormal sinus node function,atrioventricular block, and atrial and ventricular tachyarrhythmia.

Administration of the cells or compositions can be effected in one dose,continuously or intermittently throughout the course of treatment.Methods of determining the most effective means and dosage ofadministration are known to those of skill in the art and will vary withthe composition used for therapy, the purpose of the therapy and thesubject being treated. Single or multiple administrations can be carriedout with the dose level and pattern being selected by the treatingphysician. Suitable dosage formulations and methods of administering theagents are known in the art. In a further aspect, the cells andcomposition of the invention can be administered in combination withother treatments.

The cells and populations of cell are administered to the host usingmethods known in the art and described, for example, in U.S. Pat. No.6,638,369. This administration of the cells or compositions of theinvention can be done to generate an animal model of the desireddisease, disorder, or condition for experimental and screening assays.

Screening Assays

The present invention provides methods for screening various agents thatmodulate cardiac function and for studying cardiac function in vitro.For the purposes of this invention, an “agent” is intended to include,but not be limited to a biological or chemical compound such as a simpleor complex organic or inorganic molecule, a peptide, a protein (e.g.,antibody), a polynucleotide (e.g., anti-sense) or a ribozyme. A vastarray of compounds can be synthesized, for example polymers, such aspolypeptides and polynucleotides, and synthetic organic compounds basedon various core structures, and these are also included in the term“agent.” In addition, various natural sources can provide compounds forscreening, such as plant or animal extracts, and the like. It should beunderstood, although not always explicitly stated that the agent is usedalone or in combination with another agent, having the same or differentbiological activity as the agents identified by the inventive screen.

To practice the screening method in vitro, suitable cell cultures ortissue cultures containing the modified cell(s) are first provided. Whenthe agent is a composition other than a DNA or RNA, such as a smallmolecule as described above, the agent can be directly added to the cellculture or added to culture medium for addition. As is apparent to thoseskilled in the art, an “effective” a mount must be added which can beempirically determined. When agent is a polynucleotide, it can bedirectly added by use of a gene gun or electroporation. Alternatively,it can be inserted into the cell using a gene delivery vehicle or othermethod as described above. Positive and negative controls can be assayedto confirm the purported activity of the drug or other agent.

The following examples are intended to illustrate and not limit theinventions as provided herein.

Culture and Directed Differentiation

HES2 human ESC line (ESI, Singapore) was cultured and differentiated byco-culturing with the immortalized endoderm-like END2 cells aspreviously reported (Mummery, C. et al. (2003) Circulation107:2733-2740; Reubinoff, B. E. et al. (2000) Nat. Biotechnol.18:399-404). H1 human ESC line (WiCell, Madison, Wis.) was cultured anddifferentiated by embryoid body (EB) formation (Kehat, I. et al. (2001)The Journal of Clinical Investigation 108:407-414; Xu, C. et al. (2002)Circ Res. 91:501-508; He, J. Q. et al. (2003) Circ. Res. 93:32-39) asdescribed previously. Human induced pluripotent stem cells (iPSCES4skin-clone 3), a kind gift from Dr. James Thomson (Wisconsin,Madison), were also maintained as described (Yu, J. et al. (2007)Science 318:1917-1920). IPSC-CMs were derived via EB formation followingthe H1 hESC differentiation protocol. D3 murine ESCs were cultivated anddifferentiated into spontaneously beating CMs as previously described(Wobus, A. M. et al. (2002) Methods Mol. Biol. 185:127-156).

For directed differentiation (dd) of HES2 hESCs into the cardiaclineage, cardiogenic EBs (or mesodermal cardiospheres) were generatedfor 24 hours in the presence of 0.5 ng/ml BMP4 in STEMPRO 34 media asdescribed (Yang, L. et al. (2008) Nature 453:524-528). From days 1-4 theEBs were induced with 10 ng/ml BMP4, 5 ng/ml bFGF and 3 ng/ml activin Aand then from days 4-8, they were cultured in 150 ng/ml DKK1 and 10ng/ml VEGF. From day 8 onwards the EBs were maintained in the presenceof 10 ng/ml VEGF, and 5 ng/ml bFGF. Cultures were maintained in a 5%CO₂/5% O₂/90% N₂ environment for the first 10-12 days and were thentransferred into a 5% CO₂/air environment. Directed differentiationtypically results in aggregates consisting of 40-50% CMs as assessed byexpression of cardiac troponin T using FACS. For driven maturation,ddhESCs-CMs of 24-28 days were plated for electrical stimulation at 2.5V/cm with 5 msec pulse width at 1 Hz for 14 days before experiments.

Adenovirus-Mediated Gene Transfer of Single Derived-CMs

The full-length coding sequence of human Kir2.1 was cloned into themultiple-cloning site of pAd-CMV-IRES-GFP (pAd-GFP) to generatepAd-CMV-GFP-IRES-Kir2.1 (pAd-Kir2.1), where GFP allows foridentification of positively transduced cells. Adenoviruses weregenerated by Cre-lox recombination of purified ψ5 viral DNA and shuttlevector DNA as previously described (Hardy, S. et al. (1997) J. Virol.71:1842-1849). The recombinant products were plaque purified, expandedand purified by CsCl gradient, yielding concentrations on the order of10¹⁰ PFU/ml. For transduction, adenoviral particles were added to singleddhESC-CMs (20˜25 days), EB-hESC-CMs (7+14˜21 days), END2-hESC-CMs(16˜20 days), EB-iPSC-CMs (7+21˜24 days) and EB-mESC-CMs (7+4 days) at aconcentration of ˜2×10⁹ PFU (Tse, H. F. et al. (2006) Circulation114:1000-1011).

Electrophysiology

HESC- and hiPSC-CMs were dissociated into single cells with 1 mg/mlcollagenase II and plated on 0.1% gelatin-coated glass coverslips.Electrophysiological experiments were performed using whole-cellpatch-clamp technique with an Axopatch 200B amplifier and the pClamp9.2software (Axon Instruments Inc., Foster City, Calif.). Pipette solutionwas consisted of (mM): 110 K⁺ aspartate, 20 KCl, 1 MgCl₂, 0.1 Na-GTP, 5Mg-ATP, 5 Na₂-phospocreatine, 1 EGTA and 10 HEPES, with pH of 7.3. Theexternal Tyrode's solution consisted of (mM): 140 NaCl, 5 KCl, 1 CaCl₂,1 MgCl₂, 10 glucose, and 10 HEPES with pH of 7.4. Voltage- andcurrent-clamp recordings were performed at 37° C. within 24 to 48 hoursafter adenovirus transduction. ESC-CMs were categorized into pacemaker,atrial or ventricular phenotypes based on the maximum diastolicpotential (MDP), maximum rate of rise of the AP and AP duration (APD).In general, pacemaker cells always generated spontaneous APs exhibitinga more depolarized MDP, slower maximum rate of rise and the shortestAPD. Atrial cells with a triangular AP profile have a faster rate ofrise than pacemaker cells, more hyperpolarized MDP or resting membranepotential (RMP), and intermediate APD. Ventricular cells have the mosthyperpolarized MDP or RMP, a fast rate of rise and the longest APD thatexhibited a prolonged AP phase 2. Different protocols as given in insetswere employed to elicit the corresponding currents. 5 mM nifedipine, 30mM tetrodotoxin (TTX), 1 mM Ba²⁺, 30 mM ZD7288, 10 mM E4031 and 30 mMChromanol 293B were used to define the I_(CaL), I_(Na), I_(K1), I_(f),I_(Kr), and I_(Ks), respectively. All blockers were purchased fromSigma.

Formulation of Cardiac Electrophysiology Mathematical Model

Ionic currents and membrane potential of ventricular CMs were formulatedbased on an embryonic chick ventricular cell model (Krogh-Madsen, T. etal. (2005) Am. J. Physiol. Heart Circ. Physiol. 289:H398-H413) andaccording to the algorithms that have been previously reported (Azene,E. M. et al. (2005) Cardiovascular Research 67:263-273). In this model,the six ionic currents initially included based on previous reports(Boheler, K. R. et al. (2002) Circ. Res. 91:189-201) were slow inwardCa²⁺ current (I_(Ca)), slow delayed K⁺ current (I_(Ks)), rapid delayedrectifier K⁺ current (I_(Kr)), pacemaker current (I_(f)), backgroundcurrent (I_(b)), and seal-leak current (I_(seal)). The kinetics of thecurrents was derived empirically from experimental data (Krogh-Madsen,T. et al. (2005) Am. J. Physiol. Heart Circ. Physiol. 289:H398-H413).I_(K1) was absent in this base model to simulate Applicants'experimental data and subsequently manipulated to predict the effects ofKir2.1 overexpression. The computations were performed in Matlab(Mathworks, Natick, Mass.) using a variable order ordinary differentialequation solver plus a built-in backward-difference method, withrelative tolerance of 10⁻⁸ and absolute tolerance of 10⁻⁴.

Confocal Ca⁺ Imaging

Single-cell Ca²⁺ transients of Fluo-4-loaded ddhESC-CMs were recordedwith a spinning disk confocal microscope (Yokogawa CSU10) at ˜180 framesper second with a 40× microscope objective at room temperature inTyrode's solution consisting of (mM): 140 NaCl, 5 KCl, 1 MgCl₂, 2 CaCl₂,10 glucose and 10 HEPES at pH 7.4. Ca²⁺ transients were elicited by afield stimulator at 0.2 Hz and 40 V with 90 ms pulse duration. The Ca²⁺transient changes were quantified by the background subtractedfluorescent intensity changes normalized to the background subtractedbaseline fluorescence using Image J.

Quantitative PCR

mRNA was extracted from derived CMs using RNeasy Kit (Qiagen, Valencia,Calif.). cDNA were reverse transcribed from mRNA using QuantitectReverse Transcription Kit (Qiagen). Platinum SYBR Green qPCRSuperMix-UDG (Invitrogen) was used for qPCR analysis with a BioRadiCycler (BioRad, Hercules, Calif.). The expression level was normalizedrelative to GAPDH using the ΔΔC_(T) method.

Transmission Electron Microscopy

Unpaced control and electrically conditioned beating cardiospheres fromtwo separate differentiation batches were manually dissected out, thenfixed with Karnovsky's fixative at room temperature. Specimens werefurther fixed with 1% oxmium tetroxide for 90 min. After acetonedehydration, the specimen were infiltrated with epoxy resin and allowedto polymerize at 70° C. overnight. Sections 60 nm thick were stainedwith uranyl acetate. A total of 15 unpaced control and 13 electricallyconditioned cells were imaged with a FEI/Philips CM120 TEM (Philips/FEI,Endhoven, Netherlands).

Statistics

Data are shown as mean±SEM. Unpaired Student's t test or Chi-square (χ²)test were used for statistical analysis where p<0.05 was consideredstatistical significant.

Results Pro-Arrhythmic Properties of Derived CMs are Independent ofCardiogenesis Protocol and Origin of Stem Cell Lines

Single ddhESC-CMs from cardiogenic EBs differentiated for 20-25 dayswere electrophysiologically assessed by whole-cell patch-clamptechniques. The ddhESC-CMs were classified by their ability to generateAP into spontaneously firing and quiescent ddhESC-CMs, and alsoclassified into CM subtypes by the observed signature ventricular,atrial and pacemaker APs (FIG. 1A). Unlike healthy adult CMs, 26% of allddhESC-CMs displayed a high degree of automaticity or spontaneous firingof APs (FIG. 1C). While the remaining ddhESC-CMs were quiescent,stimulation could elicit a single AP characteristics of ventricular oratrial subtype (FIG. 1A, right), indicating that their excitabilityremained intact; however, these quiescent-yet-excitable ddhESC-CMsdisplayed a prominent “phase 4-like” depolarization (FIG. 1A, arrows), aknown pro-arrhythmic triggering substrate that is also not seen inmature contractile adult CMs (ten Tusscher, K. H. et al. (2006) Am. J.Physiol. Heart Circ. Physiol. 291:H1088-H1100; Wagner, M. B. et al.(2005) Pediatr. Res. 57:28-34; Wang, Y. et al. (2003) Journal ofMolecular and Cellular Cardiology 35:1083-1092). The ddhESC-CMpopulation overall exhibited a distribution of 42% ventricular, 53%atrial and 5% pacemaker CMs (FIG. 1B; n=70). Delayed-afterdepolarization (DAD) was observed in 23% of quiescent ventricularddhESC-CMs (FIG. 1D). Furthermore, the resting membrane potentials(RMPs) were significantly more depolarized (FIG. 4E) than ˜−80 mVtypical of normal adult (ten Tusscher, K. H. et al. (2006) Am. J.Physiol. Heart Circ. Physiol. 291:H1088-H1100; Wagner, M. B. et al.(2005) Pediatr. Res. 57:28-34; Wang, Y. et al. (2003) Journal ofMolecular and Cellular Cardiology 35:1083-1092) and comparable to ˜−53mV typical of immature human fetal ventricular CMs of 18 weeks.

Given the potential for myocardial repair, Applicants next focused onanalyzing ventricular ddhESC-CMs. To understand the electrophysiologicalbasis of the observed AP phenotypes, Applicants functionally profiledthe individual ionic components present (FIG. 1E and FIG. 2).Ventricular ddhESC-CMs robustly expressed I_(Na), I_(CaL) and I_(tol)(Yang, L. et al. (2008) Nature 453:524-528), which are comparable toadult ventricular CMs. I_(f) that is typically not seen in healthy adultventricular CMs (except in immature or pathophysiological states such ashypertension and heart failure) was highly expressed in both atrial andventricular ddhESC-CMs with a higher current density for the atrialrelative to ventricular subtype, but their steady-state activationcurves were identical (FIGS. 2A and 2B). I_(CaL) was present at highercurrent density in ventricular relative to atrial ddhESC-CMs withsimilar steady-state inactivation properties (FIGS. 2D and 2D). However,the inwardly rectifying K⁺ current (I_(K1)) and slow (I_(Ks)) componentsof the delayed rectifier that are present in high current density inadult CMs for repolarization were not present in ventricular ddhESC-CMs.Although not as robust as the adult counterpart, the rapid component ofthe delayed rectifying K⁺ current (I_(Kr)) was expressed in both atrialand ventricular ddhESC-CMs with a slightly higher current density in thelatter (FIG. 2F).

Inductive interactions among the three primitive germ layers figureprominently in embryogenesis (Saxen, L. (1975) Clin. Obstet. Gynecol.18:149-175), the first step of ddhESC-CMs derivation involved directedmesodermal differentiation (Yang, L. et al. (2008) Nature 453:524-528).Considering the method of induction as well as the specific hESC linemay influence the type of CMs that develops (Moore, J. C. et al. (2008)Biochem. Biophys. Res. Commun. 372:553-558), Applicants next examinedand compared electrophysiology of hESC-CMs derived by two distinctmethods: 1) endodermal induction with END2 co-culturing of the HES2(END2-hESC-CMs) (Mummery, C. et al. (2003) Circulation 107:2733-2740)and 2) EB formation of the H1 hESC lines (EB-hESC-CMs) (Kehat, I. et al.(2001) The Journal of Clinical Investigation 108:407-414). Like theddhESC-CMs, immature AP properties such as automaticity, phase 4-likedepolarization, depolarized RMP, the presence of DAD and I_(f), and theabsence of I_(K1) were all commonly observed in both END2- (FIGS. 3A-3C)and EB-hESC-CMs (FIG. 3D). Even human induced pluripotent stem cell (Yu,J. et al. (2007) Science 318:1917-1920) (iPSC)-derived CMs by EBformation behaved similarly (FIG. 1F). Taken together, these datasuggest that the immature, pro-arrhythmic AP properties are genuinedevelopmental hallmarks of early stage human CMs derived in vitro.Unfortunately, derived CMs failed to mature further even after culturingfor >100 days (Sartiani, L. et al. (2007) Stem Cells 25:1136-1144).These data hint at the intriguing possibility that a crucial physicalcomponent that drives hESC-CM maturation in vivo is missing in theconventional method of in vitro culture.

I_(k1) is a Major Mechanistic Contributor to the Immature AP of hESC-CMs

In normal fetal CM development, one of the major electrophysiologicalchanges is a progressive increase in I_(K1) and concomitant reduction inI_(f) (Nass, R. D. et al. (2008) Nat. Clin. Pract. Cardiovasc. Med.5:196-207). In heart failure, the fetal gene program is re-initiated inadult CMs to cause electrical remodeling (Beuckelmann, D. J. et al.(1993) Circulation Research 73:379-385), such that I_(K1) becomesdown-regulated whereas I_(f) is reciprocally upregulated, therebypredisposing the afflicted individuals to lethal arrhythmias. Along thisline, Applicants data collectively hint at three possible mechanisms forthe immature AP phenotypes: the presence of I_(f), the absence ofI_(K1), or both. To understand the basis of electrical immaturities andthereby develop a strategy for maturing and eliminating the undesirableproarrhythmic traits of derived CMs, Applicants next performed an insilico analysis of the AP profiles (FIG. 4A) (Krogh-Madsen, T. et al.(2005) Am. J. Physiol. Heart Circ. Physiol. 289:H398-H413; Azene, E. M.et al. (2005) Cardiovascular Research 67:263-273). By incorporating allthe ionic components identified (FIG. 1E), Applicants ventricular modelsufficed to reproduce the experimentally determined AP parameters. Whenthe maximum conductance of I_(K1) (G_(K1)) was increased from 0, theexperimentally observed level, to 3.6 nS (i.e., ⅛ of that of adultventricular cells) and subsequently to 7.2 nS, the firing rate decreasedaccordingly. When GK1 was 10.8 nS or higher, the spontaneous firingceased along with RMP hyperpolarized to the adult level; theseI_(K1)-silenced cells remained excitable and could generate a normalventricular AP when triggered by a stimulus. Silencing could not beachieved by I_(f) suppression alone; thus, according to Applicants'model, the absence of I_(K1) appears to be a major mechanisticcontributor of pro-arrhythmic automaticity.

To experimentally test Applicants' mathematical modeling results,Applicants generated the recombinant adenovirus Ad-CMV-GFP-IRES-Kir2.1(or Ad-Kir2.1) to mediate the expression of Kir2.1 channels thatunderlie I_(K1) (Yang, J. et al. (1995) Neuron. 15:1441-1447). Incontrast to untransduced or Ad-GFP-transduced I_(K1)-negative control,Ad-Kir2.1-transduced ventricular ddhESC- (n=7) and END2-hESC- (n=13) CMsrobustly expressed Ba²⁺-sensitive I_(K1) (FIGS. 4B, 4C and 4G). Moreimportantly, the percentages of quiescent ventricular ddhESC- andEND2-hESC-CMs dramatically increased to 100% (FIG. 4D; p<0.05) withtheir RMP significantly hyperpolarized (p<0.05, FIG. 4E) to the adultlevel (p>0.05). Ad-Kir2.1-silenced cells remained excitable and couldgenerate a single AP upon stimulation, but most importantly, the phase4-like depolarization was completely eliminated (FIGS. 4B and 4C,arrow). Interestingly, Ad-Kir2.1-silenced ventricular ddhESC-CMsreverted back to the spontaneously firing phenotype upon the addition ofan I_(K1) blocker, Ba²⁺ (FIG. 4F). Kir2.1 expression also sufficed tomature the AP profiles of atrial ddhESC- and END2-hESC-CMs (FIGS.5A-5D).

Similar to their human counterparts, murine (m) ventricular and atrialESC-CMs exhibited comparably immature electrophysiological properties(FIGS. 8A and 8B) that could likewise be rendered adult-like by Kir2.1expression (FIGS. 9A-9F and 10A-10E). These data collectively indicatethat Applicants' observations of electrophysiological immaturities inthe derived CMs were a general phenomenon and not cell line-, species-or protocol-dependent.

Physiological Pacing Facilitates Maturation of hESC-CMs In Vitro

Despite Applicants' achievement of electrical maturation,Ad-Kir2.1-matured CMs continued to exhibit immature Ca²⁺-handlingproperties with smaller peak Ca²⁺ transient amplitudes and slow kineticsthat are comparable to the control group (FIG. 11) (Liu, J. et al.(2007) Stem Cells 25:3038-3044). Indeed, the expression levels ofsarcomeric genes, such as MHCα, MHCβ, MLC2a and MLC2v, of theKir2.1-silenced ddhESC-CMs even became significantly suppressed relativeto control ddhESC-CMs (p<0.05; FIG. 4G). Given that sarcomeric proteinsof the developing heart are known to respond to changes in contractions(Razeghi, P. et al. (2001) Circulation 104:2923-2931), Applicants'observation could be attributed to the lack of spontaneous beatingactivities of ddhESC-CMs in culture after Kir2.1-induced cessation ofactive contractions, which in turn led to the down-regulation of thecontractile apparatus. Indeed in developing neurons, Kir2.1 expressioncan alter their excitability by escalating in response to extrinsicexcitation via an activity-dependent mechanism to mediate synapticplasticity (Razeghi, P. et al. (2001) Circulation 104:2923-2931;Burrone, J. et al. (2002) Nature 420:414-418). To test if electricalactivity likewise affects Kir2.1 expression in developing ddhESC-CMs,Applicants' cells mimicked endogenous pacing of adult heart bysystematically field-stimulating cells in culture with electrical pulseof 2.5 V/cm at 5 msec pulse duration and 1 Hz frequency for 14 days.Applicants hypothesized that 1) electrical conditioning of ddhESCCMssuffices to promote in vitro electrophysiological maturation, and that2) the pacing-induced regular contractions can facilitate maturation ofCa²⁺-handling and contractile properties in a manner similar to thefetal heart development. Applicants' experiments showed that this wasindeed the case. Both electrically conditioned ventricular and atrialddhESC-CMs were 100% quiescent with absence of phase 4-depolarization(FIG. 6A, arrows; n=11). Moreover, the RMPs of the atrial andventricular ddhESC-CMs were significantly hyperpolarized (p<0.05). Sucha mature AP phenotype was never observed in >150 time-matchedun-stimulated control hESC-CMs that Applicants had recorded.Consistently, Kir2.1 expression became elevated (FIG. 6A). Compared tounpaced controls, electrical conditioning similarly augmented both theelectrically induced Ca²⁺ transient amplitude (FIG. 6B; p<0.05) andsarcoplasmic reticulum Ca²⁺ load as shown by caffeine induced Ca²⁺transients (FIG. 6C; p<0.05) of ddhESC-CMs. Consistently, the expressionlevels of Ca²⁺-handling proteins typically present in adult CMs butabsent or barely expressed in hESC-CMs such as calsequestrin (CSQ),junctin (Jct), and triadin (Trdn) (Liu, J. et al. (2007) Stem Cells25:3038-3044) as well as the t-tubule biogenesis proteins caveolin-3(Cav3) and amphiphysin-2 (Amp2) all increased (FIGS. 6D-6E). Consistentwith an increase of ddhESC-CMs with maturing ventricular phenotype,electrical conditioning also resulted in a decrease of the atrialnatriuretic factor (ANF) (FIG. 6E). More importantly, the contractileproteins MHCα, MHCβ, MLC2a and MLC2v of electrically conditionedddhESCCMs were significantly up-regulated (FIG. 6F) relative to thecontrol cells. In addition, the myofilaments became consistently morestructured and organized as shown by transmission electron microscopy(TEM) (FIG. 6G, FIGS. 12A and 12B for high resolution images),signifying contractile maturation. The increase in electricalconditioning-induced MLC2v expression could be attributed to a ˜43%increase in the number of ventricular CMs as assessed by the number ofMLC2v-positive cells relative to the tropomyosin-positive cells using alaser scanning cytometer. Similar to human cells, electricalconditioning likewise increased the expression of MLC2v and theventricular derivatives in mESC-CMs as demonstrated by flow cytometricanalysis of lentivirus generated EF1a-GFP-MLC2v-DsRed mESC line whereventricular CMs were identified by their expression of DsRed under theMLC2v promoter (FIGS. 13A and 13B).

Time-, Age- and Frequency-Dependence of the Pro-Maturation Effect ofElectrical Conditioning

Applicants investigated the roles of the duration of cell exposure tofield stimulation, age of hESC-VCMs when subjected to electricalconditioning and stimulation frequency (0.5 Hz-2 Hz) in thepro-maturation effect of electrical conditioning on hESC-VCMs. Whencompared to day 0 control, Kir2.1 expression of early-stage (20-25 daysold) hESC-CMs was not different after electrical stimulation (at 1 Hz)for 7 days but became time-dependently increased at day 14 and furtherelevated at day 21 by ˜1.7-fold (FIG. 7A). When stimulated at 1 Hz,later-stage (40-50 days old) hESC-CMs displayed ˜3-fold higherexpression levels but such increases were not statistically differentbetween days 7 and 21, unlike the early-stage counterpart (FIG. 7B).FIG. 7C shows that when pacing of early-stage hESC-CMs was ceased afterstimulating for 14 days, Kir2.1 expression declined after 7 days,indicating that continuous stimulation was needed to maintain or augmentthe gained expression. Furthermore, electrical conditioning at 1 Hz ofearly-stage hESC-CMs for 14 days led to 2.3- and 1.9-fold higher Kir2.1expression than 0.5 and 3 Hz, respectively. Collectively, these resultsindicate that our mechanism-based approach sufficed to induce maturationin ddhESC-derived CMs.

Discussion

Although conceptually promising as an unlimited source, a number offundamental hurdles need to be overcome before the use of derived CMsfrom hESCs or patient-specific iPSCs for clinical and other applicationscan be realized. It is well accepted that hESC-CMs areelectrophysiologically immature, but the underlying mechanisms remainlargely unknown. Indeed, multiple ionic currents in hESC-CMs behavedifferently from those of adult. In this study, various pluripotent stemcell line-derived CMs were investigated as well as different methods ofcardiogenic differentiation methods. This data collectively demonstratethat the observed immature electrophysiological properties areindependent of the specific pluripotent stem cell types and lines,differentiation protocols as well as species, suggesting that a crucialenvironmental cue might be missing in the in vitro culturing systemthereby leading to the artifactual developmental arrest.

Critical Role of I_(K1) in the Maturation of hESC-CMs

For maintaining an electrophysiologically stable phenotype with ahyperpolarized resting membrane potential close to the reversalpotential of K⁺, an I_(K1) magnitude of at least 50% that of adultventricular CMs is required. Although the I_(K1) magnitude of hESC-CMshas been shown to increase with prolonged culture of 100 dayspost-differentiation (Sartiani, L. et al. (2007) Stem Cells25:1136-1144), the current density in these aged cells is still belowthe threshold for attaining a mature ventricular electrical phenotype(Bailly, P. et al. (1998) Circulation 98:2753-2759). Therefore, it isimpractical to maintain the cells for more than 100 days to achieve onlya slight, if not entirely insignificant, improvement for clinical andother applications. One goal of the present study is to designstrategies to effectively drive their maturation. By first exploring thebasis that underlies the observed electrophysiological immaturities,Applicants identified the lack of I_(K1) as one of the major mechanisticcontributors of cellular automaticity. Forced expression of Kir2.1channels in hESC-CMs alone sufficed to completely eliminate all theimmature and proarrhythmic traits and thereby reproducing the adult APphenotype. Although Ad-Kir2.1-matured hESC-CMs “corrected” automaticityimmaturity, these cells continued to exhibit immature Ca²⁺-handlingproperties. Moreover, these quiescent cells have reduced expression ofcontractile proteins, MHCa, MHCb, MLC2a and MLC2v, as quantified byqPCR.

Mechanisms Underlying the Increased Maturation by Electrical Stimulation

Considering the data and the tight E-C coupling betweenelectrophysiological and Ca²⁺-handling properties, Applicantshypothesized that rhythmic electrical conditioning of hESC-CMs in vitroto mimic endogenous pacing may achieve E-C maturation through: 1)chronic pacing with field-stimulation may induce ion channel expressionchanges, 2) pacing-induced active contractions of electrically silencedhESC-CMs with mature APs can in turn facilitate Ca²⁺-handling andcontractile maturation in a manner similar to the normal fetal heartdevelopment. Of note, although immature hESC-CMs do spontaneously fireAPs and contract and without being bound by theory, Applicants submitthat these activities may be too weak, unsustained and sporadic incomparison to the physiological levels for effective facilitation ofmaturation in vitro. Electrical conditioning robustly led to manyaspects of cellular maturation of hESC-CMs, includingelectrophysiological maturation without phase 4-depolarization similarto Kir2.1 gene transfer, Ca²⁺-handing maturation with increased peakCa²⁺ transient amplitude and SR Ca²⁺ load, structured organization ofmyofilaments as well as an up-regulation of contractile and t-tubulebiogenesis proteins. Although the short-term electrical conditioning didmature numerous aspects of CM functions relative to control, the restingmembrane potential of electrically conditioned ventricular ddhESC-CMs isstill about 10 mV more depolarized, with smaller peak Ca²⁺ transientamplitude and slower kinetics, than that of adult ventricular CMs.Future studies will be required to optimize physical parameters ofelectrical conditioning such as the voltage pulse, pulse duration andfrequency for maximized facilitating effect.

These observations were qualitatively similar in some ways to what havebeen observed in rodent CMs. For instance, neonatal rat and ESC-like P19mouse embryonic carcinoma cell (ECC)-derived CMs stimulated to activelycontract by pulsed electric field can self-align sarcomeres leading toimproved contractility (Au, H. T. H. et al. (2007) Biomaterials28(29):4277-4293; Radisic, M. et al. (2004) Proc. Nat. Acad. Sci. USA101:18129-18134). Without being bound by theory, Applicants speculatethat the rhythmic field-stimulations of hESC-CMs that otherwiseintrinsically beat at low and/or irregular frequency may result incyclic increase in intracellular Ca²⁺ concentration, which can increasetheir averaged Ca²⁺ residence time. Such an increase in intracellularCa²⁺ may in turn contribute to the maturation effect observed via thesecond messenger system (e.g., by activating the calcineurin-NFATpathway) (Kassiri, Z. et al. (2002) Circulation Research 90:578-585;Lammerding, J. et al. (2004) Annals of the New York Academy of Sciences1015:53-70; Lebeche, D. et al. (2004) Circulation 110:3435-3443; Zobel,C. et al. (2002) Circulation 106:2385-2391).

Significance of Cell Maturation In Vitro Prior to Transplantation

More immature stem cell-derived CMs has been suggested to be moreischemic-resistant and tolerant to the hypoxic environment aftertransplantation (Boheler, K. R. et al. (2011) Stem Cells Int.2011:214203). The in vivo environment has also been demonstrated toinduce electrophysiological maturation of immature CMs comparable to thehost cells after transplantation but only for those that have fullyintegrated with the host cells (Halbach, M. et al. (2007) Circ Res.101:484-492). Those transplanted CMs without electrical coupling remainselectrophysiologically immature, which can be arrhythmogenic asdemonstrated herein. Therefore, it is also imperative to find a balancebetween safety and efficacy by defining what maturation status is bestfor in vivo transplantation since immature and mature hESC-CMs likelyrespond differently to ischemic and hypoxic environment. Eliminatingintrinsic triggers for arrhythmia of the hESC-CMs to ensure noadditional electrophysiological complications from their presence is ofcritical importance. The safety of these cells must be addressed beforepondering their potential benefits to the recipient.

The strategy reported here, developed by first obtaining anunderstanding of the cellular differences and the underlying mechanisms,offers a simple non-genetic way to facilitate the E-C maturation ofotherwise developmentally arrested derived CMs, thereby eliminatingsignificant undesirable immature pro-arrhythmogenic traits. Furthermore,the successful use of derived CMs as human heart disease models andcardiotoxicity screening tools depends on their ability to recapitulatethe properties of their adult counterparts. In combination with otheradvances in directed differentiation and cardiovascular progenitoridentification (Yang, L. et al. (2008) Nature 453:524-528; Bu, L. et al.(2009) Nature 460:113-117; Moretti, A. et al. (2006) Cell127:1151-1165), this approach can facilitate the clinical translationand enable more accurate human heart disease modeling, drug discoveryand cardiotoxicity screening. The modified cells of this disclosure haveno native counterparts in the body; their cardiac derivatives and themodified cells as described herein are also not identical (e.g., smallerin size, Ca transient properties are different, certain key cardiacgenes are differentially expressed) to what are seen in nature.

It is to be understood that while the invention has been described inconjunction with the above embodiments, that the foregoing descriptionand examples are intended to illustrate and not limit the scope of theinvention. Other aspects, advantages and modifications within the scopeof the invention will be apparent to those skilled in the art to whichthe invention pertains.

1. A method for inducing expression of the Kir2.1 gene in a cell lackingfunctional Kir2.1 gene expression in the absence of genetic manipulationof the cell, comprising administering to the cell an effective amount ofelectrical pacing, thereby inducing expression of Kir2.1 gene in thecell.
 2. A method for inducing an electrophysiologically maturephenotype in a cell that is not terminally differentiated orelectrophysiologically mature, comprising administering to the cell aneffective amount of electrical pacing, thereby inducing theelectrophysiologically mature phenotype in the cell.
 3. A method forpreparing an electrophysiologically mature cardiomyocyte in a cell thatis not terminally differentiated or electrophysiologically mature,comprising administering to the cell an effective amount of electricalpacing, thereby preparing an electrophysiologically maturecardiomyocyte.
 4. The method of claim 3, wherein theelectrophysiologicially mature phenotype comprises a cardiomyocyte thatexpresses one or more of an increase in one or more of the contractileproteins selected from MHCα, MHCβ, MLC2a, MLC2v and cardiac ion channelpump proteins.
 5. The method of claim 3, wherein the method excludesgenetic manipulation of the cell to enhance or increase Kir2.1expression.
 6. The method of claim 2, wherein the cell comprises one ormore of an embryonic stem cell, a pluriopotent stem cell, an embryoidbody, a mesodermal cardiosphere or an induced pluripotent stem cell. 7.The method of claim 2, wherein the electrophysiologically maturecardiomyocyte produced by the method expresses upregulated contractileproteins.
 8. The method of claim 3, wherein the electrophysiologicallymature cardiomyocyte produced by the method expresses upregulatedcontractile proteins.
 9. The method of claim 2, wherein the cell that isnot electrophysiologically mature cardiomyocyte comprises a cardiac cellof the group of a ventricular cell or an atrial cell.
 10. The method ofclaim 3, wherein the cell that is not electrophysiologically maturecardiomyocyte comprises a cardiac cell of the group of a ventricularcell or an atrial cell.
 11. The method of claim 2, wherein theelectrical pacing comprises a pulsed electrical current of from about2.0 to about 3.0 v/cm for about 2 msec to about 8 msec, at a width ofabout 0.5 HZ to about 1.5 HZ.
 12. An isolated cell produced by themethod of claim 2, wherein the cell is electrophysically mature and isnot terminally differentiated.
 13. An expanded population substantiallyhomogenous population of cells of claim
 12. 14. A clonal population ofcells of claim
 13. 15. A composition comprising the isolated cell ofclaim 12 and a carrier.
 16. A composition comprising an expandedpopulation of claim 13 and a carrier.
 17. A composition comprising anexpanded population of claim 14 and a carrier.
 18. The composition ofclaim 15, wherein the carrier is one or more of a pharmaceuticallyacceptable carrier or a biocompatible scaffold.
 19. A method fortreating a subject in need thereof, comprising administering to thesubject an effective amount of the cell of claim
 12. 20. A method fortreating a subject in need thereof, comprising administering to thesubject an effective amount of the clonal population of claim 14.